Firefox Theming

Source of truth

Firefox ships CSS inside omni.ja (a zip file). Always extract and read source files before writing or auditing userChrome rules.

Extract source

OMNI=$(ls /nix/store/*firefox-unwrapped*/lib/firefox/browser/omni.ja 2>/dev/null | head -1)
OMNI_TK=$(ls /nix/store/*firefox-unwrapped*/lib/firefox/omni.ja 2>/dev/null | head -1)
DEST=/tmp/ff-omni

mkdir -p "$DEST" && cd "$DEST"

# CSS
unzip -o "$OMNI" "chrome/browser/skin/classic/browser/*.css" "chrome/browser/skin/classic/browser/**/*.css" 2>/dev/null
unzip -o "$OMNI_TK" "chrome/toolkit/skin/classic/global/*.css" "chrome/toolkit/skin/classic/global/**/*.css" 2>/dev/null

# DevTools
unzip -o "$OMNI" "chrome/devtools/skin/*.css" 2>/dev/null

# XUL/HTML
unzip -o "$OMNI" "chrome/browser/content/browser/browser.xhtml" 2>/dev/null

Verify element existence: grep 'id="element-id"' $DEST/chrome/browser/content/browser/browser.xhtml.

Reference

references/source-map.md — complete variable/element/class reference for:

• Browser CSS (chrome/browser/skin/classic/browser/): browser-colors, browser-shared, toolbarbuttons, urlbar-searchbar, urlbarView, tabs, sidebar, contextmenu, identity-block, panelUI-shared, unified-extensions, downloads, findbar, and more
• Toolkit CSS (chrome/toolkit/skin/classic/global/): global-shared, popup, menu, findbar, in-content/common-shared, design-system/tokens-shared
• DevTools CSS (chrome/devtools/skin/): variables.css theme variables
• browser.xhtml: structural IDs, urlbar IDs, sidebar IDs, toolbar button IDs, key classes, non-existent IDs

Consult the source-map first. Extract and read the actual source file when the map lacks detail.

Workflow

1. Consult source-map.md for the section being edited
2. Extract source CSS from omni.ja when needed
3. Read the relevant source file
4. Verify every selector targets elements/classes/IDs that exist in source CSS or browser.xhtml
5. Verify every CSS variable override matches a variable defined in source
6. Use variables over direct property overrides when the source uses variables
7. Remove rules that match nothing

File placement

Target	File	Selector scope
Browser chrome	userChrome.css	:root, #navigator-toolbox, etc.
Web content pages	userContent.css	@-moz-document rules
DevTools	userContent.css	:root.theme-dark / :root.theme-light

DevTools theming

Override DevTools variables in userContent.css using :root.theme-dark (or :root.theme-light).

• Do NOT use @-moz-document url-prefix("chrome://devtools/content/") — doesn't match DevTools documents
• Do NOT put DevTools overrides in userChrome.css — DevTools runs in a separate iframe
• Variables are in chrome/devtools/skin/variables.css
• Reference: one-monokai-firefox-devtools

Search mode indicators

Two systems exist. Hide both when removing the chip:

• Legacy: #urlbar-search-mode-indicator — shown via #urlbar[searchmode] > .urlbar-input-container > #urlbar-search-mode-indicator
• Scotch Bonnet: #searchmode-switcher-chicklet — shown via #urlbar[searchmode] #searchmode-switcher-chicklet (gated behind browser.urlbar.scotchBonnet.enableOverride or browser.urlbar.searchModeSwitcher.featureGate)
• Related: #urlbar-searchmode-switcher, #searchmode-switcher-title, #searchmode-switcher-close, #searchmode-switcher-dropmarker

When CSS doesn't work

Search online for working reference implementations before guessing. One working example beats ten theories. The MrOtherGuy/firefox-csshacks repo is the canonical reference for userChrome hacks.

Tabs below content

Use CSS grid on #main-window > body, NOT order / -moz-box-ordinal-group:

@media not -moz-pref("sidebar.verticalTabs") {
  #main-window > body {
    display: grid !important;
    grid-template-rows: repeat(8, max-content) 1fr;
    grid-auto-rows: auto;
  }
  #navigator-toolbox {
    display: contents;
  }
  #main-window #browser {
    grid-row: 9 / 10;
  }
  #TabsToolbar {
    grid-row: 10 / 11;
  }
  .browser-toolbar {
    background: inherit;
    background-attachment: fixed;
  }
}

Why:

• order / -moz-box-ordinal-group moves the entire #navigator-toolbox — grid lets each toolbar land in its own row
• -moz-box-ordinal-group is legacy XUL layout, actively being removed
• display: contents on #navigator-toolbox is required so its children participate in the grid

Source: firefox-csshacks/chrome/tabs_below_content_v2.css

Hiding tabs with a single tab

Critical rules beyond just collapsing the tab:

#TabsToolbar {
  min-height: 0px !important;
}
#tabbrowser-tabs,
#pinned-tabs-container,
#tabbrowser-arrowscrollbox {
  min-height: 0 !important;
}
.accessibility-indicator,
.private-browsing-indicator {
  height: unset !important;
}
.accessibility-indicator > hbox {
  padding-block: 0 !important;
}

/* Match both :only-of-type AND attribute-based selectors for tab groups/hidden tabs */
.tabbrowser-tab:only-of-type,
.tabbrowser-tab[first-visible-tab="true"][last-visible-tab="true"] {
  visibility: collapse !important;
  min-height: 0 !important;
  height: 0;
}

/* Contain periphery so it can't hold toolbar open */
#tabbrowser-arrowscrollbox-periphery,
#private-browsing-indicator-with-label,
#TabsToolbar > .titlebar-buttonbox-container {
  contain: strict;
  contain-intrinsic-height: 0px;
}

Why:

• :only-of-type alone fails when tabs are hidden by extensions or tab groups — [first-visible-tab][last-visible-tab] covers those cases
• #pinned-tabs-container can force minimum height even when all tabs collapse
• .accessibility-indicator, .private-browsing-indicator, .titlebar-buttonbox-container, #tabbrowser-arrowscrollbox-periphery all hold the toolbar open without contain: strict
• These periphery elements are the most common cause of persistent gaps

Source: firefox-csshacks/chrome/hide_tabs_with_one_tab.css

Removing toolbox borders and separators

Three rules required — setting --chrome-content-separator-color: transparent alone is insufficient:

:root[sizemode="normal"] {
  border-top: none !important;
}
#navigator-toolbox::after {
  content: none !important;
}
#navigator-toolbox {
  border-bottom: none !important;
}

The ::after pseudo-element on #navigator-toolbox draws a bottom border that CSS variables do not control.

Source: firefox-csshacks/chrome/hide_toolbox_top_bottom_borders.css

Tab background direct overrides

CSS variables like --tab-border-radius are not consumed by all internal tab styles. Apply direct overrides:

.tab-background {
  border-radius: 0 !important;
  box-shadow: none !important;
  border-top: 0 !important;
  outline: none !important;
}

Source: firefox-csshacks/chrome/non_floating_sharp_tabs.css

Preventing tab animation interference

Firefox's built-in tab animation system can fight visibility collapse:

#TabsToolbar {
  will-change: unset !important;
  transition: none !important;
  opacity: 1 !important;
}

Source: firefox-csshacks/chrome/non_floating_sharp_tabs.css

Autohide entire toolbox

Hide all toolbars, show on hover or urlbar focus. Uses rotateX transform (GPU-accelerated, no layout shift):

#navigator-toolbox {
  position: fixed !important;
  width: 100vw;
  z-index: 10 !important;
  transition: transform 82ms ease-in-out 600ms !important;
  transform: rotateX(89.9deg);
  transform-origin: top;
  opacity: 0;
}
#navigator-toolbox:is(:hover, :focus-within) {
  transition-delay: 0ms !important;
  transform: rotateX(0deg);
  opacity: 1;
}

The #urlbar[popover] also needs separate hide/show since it's a popover:

#urlbar[popover] {
  opacity: 0;
  pointer-events: none;
  transition:
    transform 82ms ease-in-out 600ms,
    opacity 0ms 682ms;
  transform: translateY(
    calc(0px - var(--tab-min-height) - var(--urlbar-container-height))
  );
}
#urlbar-container > #urlbar[popover]:is([focused], [open]) {
  opacity: 1;
  pointer-events: auto;
  transition-delay: 0ms;
  transform: translateY(0);
}

Keep toolbox visible when any popup is open:

#mainPopupSet:has(> [panelopen]:not(#tab-preview-panel)) ~ #navigator-toolbox {
  transition-delay: 0ms !important;
  transform: rotateX(0deg);
  opacity: 1;
}

Source: firefox-csshacks/chrome/autohide_toolbox.css

Autohide nav-bar only

Show nav-bar overlaid on tabs when urlbar is focused. Uses grid to stack toolbars:

:root:not([customizing]) #navigator-toolbox {
  display: grid;
  grid-template-rows: auto;
}
:root:not([customizing]) #navigator-toolbox > .browser-toolbar {
  grid-area: 1/1;
}

:root[sessionrestored] #nav-bar:not([customizing]) {
  transform: rotateX(89.9deg);
  transition:
    transform 67ms linear,
    opacity 0ms linear 67ms !important;
  opacity: 0;
  z-index: 3;
}
:root[sessionrestored] #nav-bar:focus-within {
  transform: rotateX(0deg);
  opacity: 1;
  transition-delay: 0ms, 0ms !important;
}

Source: firefox-csshacks/chrome/show_navbar_on_focus_only.css

Autohide tabs toolbar

Hides tabs toolbar, shows on hover. Uses negative margin + transition:

:root {
  --uc-tabs-hide-animation-duration: 48ms;
  --uc-tabs-hide-animation-delay: 200ms;
}
#TabsToolbar:not([customizing]) {
  visibility: hidden;
  position: relative;
  z-index: 1;
  transition:
    visibility 0ms linear var(--uc-tabs-hide-animation-delay),
    margin-bottom var(--uc-tabs-hide-animation-duration) ease-out
      var(--uc-tabs-hide-animation-delay) !important;
}
#navigator-toolbox:has(> :is(#toolbar-menubar, #TabsToolbar):hover)
  > #TabsToolbar {
  visibility: visible;
  margin-bottom: 0px;
  transition-delay: 0ms, 0ms !important;
}

Source: firefox-csshacks/chrome/autohide_tabstoolbar_v2.css

Autohide bookmarks toolbar

Uses rotateX with configurable delay:

#PersonalToolbar {
  --uc-bm-height: 20px;
  --uc-bm-padding: 4px;
  --uc-autohide-toolbar-delay: 600ms;
}
#PersonalToolbar:not([customizing]) {
  position: relative;
  margin-bottom: calc(-1px - var(--uc-bm-height) - 2 * var(--uc-bm-padding));
  transform: rotateX(90deg);
  transform-origin: top;
  transition: transform 135ms linear var(--uc-autohide-toolbar-delay) !important;
  z-index: 1;
}
#navigator-toolbox:hover > #PersonalToolbar {
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

Source: firefox-csshacks/chrome/autohide_bookmarks_toolbar.css

Autohide sidebar

Collapses sidebar to narrow strip, expands on hover:

#sidebar-box {
  --uc-sidebar-width: 40px;
  --uc-sidebar-hover-width: 210px;
  --uc-autohide-sidebar-delay: 600ms;
  --uc-autohide-transition-duration: 115ms;
  min-width: var(--uc-sidebar-width) !important;
  width: var(--uc-sidebar-width) !important;
  max-width: var(--uc-sidebar-width) !important;
  z-index: 3;
  position: relative;
}
#sidebar-header,
#sidebar {
  transition: min-width var(--uc-autohide-transition-duration) linear
    var(--uc-autohide-sidebar-delay) !important;
  min-width: var(--uc-sidebar-width) !important;
  will-change: min-width;
}
#sidebar-box:hover > #sidebar-header,
#sidebar-box:hover > #sidebar {
  min-width: var(--uc-sidebar-hover-width) !important;
  transition-delay: 0ms !important;
}
#sidebar-splitter {
  display: none;
}

Source: firefox-csshacks/chrome/autohide_sidebar.css

Hide tabs toolbar entirely

For use with tree-style-tab or native vertical tabs. Moves window controls to nav-bar:

@media not -moz-pref("sidebar.verticalTabs") {
  #TabsToolbar:not([customizing]) {
    visibility: collapse;
  }
  :root[customtitlebar]
    #toolbar-menubar:is([autohide=""], [autohide="true"])
    ~ #nav-bar {
    > .titlebar-buttonbox-container {
      display: flex !important;
    }
    :root[sizemode="normal"] & {
      > .titlebar-spacer {
        display: flex !important;
      }
    }
  }
}
@media -moz-pref("sidebar.verticalTabs") {
  #sidebar-launcher-splitter,
  #sidebar-main {
    visibility: collapse;
  }
}

Source: firefox-csshacks/chrome/hide_tabs_toolbar_v2.css

Tabs on bottom (within toolbox)

Reorders TabsToolbar below nav-bar without moving below content. Uses order:

@media not -moz-pref("sidebar.verticalTabs") {
  .global-notificationbox,
  #tab-notification-deck,
  #notifications-toolbar,
  #TabsToolbar {
    order: 1;
  }
  #TabsToolbar > :is(.titlebar-spacer, .titlebar-buttonbox-container) {
    display: none;
  }
  :root[customtitlebar]
    #toolbar-menubar:is([autohide=""], [autohide="true"], [collapsed])
    ~ #nav-bar {
    > .titlebar-buttonbox-container {
      display: flex !important;
    }
  }
}

Source: firefox-csshacks/chrome/tabs_on_bottom_v2.css

All toolbars below content

Moves entire toolbox below content. Uses display: contents + order:

#navigator-toolbox {
  display: contents;
  --uc-navbar-height: 40px;
}
#main-window > body > #browser,
.global-notificationbox,
#tab-notification-deck,
#notifications-toolbar,
#toolbar-menubar {
  order: -1;
}

Urlbar breakout must flip direction when toolbars are below:

#urlbar[breakout][breakout-extend] {
  display: flex !important;
  flex-direction: column-reverse !important;
  transform: translateY(calc(var(--urlbar-container-height) - 100%));
}
.urlbarView-body-inner {
  border-top-style: none !important;
}

Source: firefox-csshacks/chrome/toolbars_below_content_v2.css

One-line toolbar (tabs + nav-bar side by side)

Uses CSS grid with fractional columns:

@media not -moz-pref("sidebar.verticalTabs") {
  :root:not([chromehidden~="toolbar"]) #navigator-toolbox {
    display: grid;
    grid-template-columns: 6fr 4fr;
  }
  #toolbar-menubar,
  #PersonalToolbar,
  .global-notificationbox {
    grid-column: 1/3;
  }
  #TabsToolbar,
  #nav-bar {
    grid-row: 2/3;
  }
  #nav-bar {
    border-top: none !important;
  }
}

Source: firefox-csshacks/chrome/oneline_toolbar.css

Combined tabs + nav-bar

Tabs share space with nav-bar using flex-wrap:

#navigator-toolbox {
  display: flex;
  flex-direction: row-reverse;
  flex-wrap: wrap;
}
#nav-bar,
#PersonalToolbar {
  flex-grow: 1000;
}
#urlbar-container {
  min-width: 250px !important;
}
#urlbar[open]:focus-within {
  min-width: var(--uc-urlbar-min-width, none) !important;
}

Source: firefox-csshacks/chrome/combined_tabs_and_main_toolbars.css

Vertical tabs (sidebar-style)

Uses position: absolute on #tabbrowser-tabs with margin on content:

:root:not([customizing]) {
  --uc-vertical-tabs-width: 220px;
  --uc-navbar-height: 40px;
}
#PersonalToolbar,
#main-window:not([inDOMFullscreen]) > body > #browser {
  margin-left: var(--uc-vertical-tabs-width);
}
:root:not([customizing]) #tabbrowser-tabs {
  position: absolute !important;
  height: 100vh;
  left: 0;
  padding-top: var(--uc-navbar-height);
  width: var(--uc-vertical-tabs-width);
  background-color: var(--toolbar-bgcolor);
  contain: size;
}

Source: firefox-csshacks/chrome/vertical_tabs.css

Multi-row tabs

Flexbox wrap with scroll:

:root {
  --multirow-n-rows: 3;
  --multirow-tab-min-width: 100px;
}
#tabbrowser-tabs[orient="horizontal"] {
  min-height: unset !important;
  flex-wrap: wrap;
  overflow-y: auto;
  overflow-x: hidden;
  max-height: calc(
    (var(--tab-min-height) + 2 * var(--tab-block-margin, 0px)) *
      var(--multirow-n-rows)
  );
  scrollbar-width: thin;
  scroll-snap-type: y mandatory;
}
.tabbrowser-tab {
  scroll-snap-align: start;
}
#tabbrowser-tabs .tabbrowser-tab[pinned] {
  position: static !important;
}

Tab reordering does not work with multi-row. Hide scroll buttons and spacer:

#scrollbutton-up,
#scrollbutton-down {
  display: none;
}
#tabbrowser-arrowscrollbox > spacer {
  display: none !important;
}

Source: firefox-csshacks/chrome/multi-row_tabs.css

Nav-bar below content

Uses position: fixed at bottom. Must use -webkit-box display (not flex) to avoid breaking extension menus:

#browser {
  margin-bottom: var(--uc-bottom-toolbar-height, 0px);
}
#nav-bar {
  position: fixed !important;
  bottom: 0px;
  display: -webkit-box;
  width: 100%;
  z-index: 1;
}
#nav-bar-customization-target {
  -webkit-box-flex: 1;
}

Urlbar breakout flips upward:

#urlbar[breakout][breakout-extend] {
  display: flex !important;
  flex-direction: column-reverse !important;
  bottom: 0px !important;
  top: auto !important;
}

Source: firefox-csshacks/chrome/navbar_below_content.css

Floating findbar

Overlays findbar at top of content area instead of pushing content down:

findbar {
  order: -1;
  margin-bottom: -33px;
  position: relative;
  border-top: none !important;
  padding: 0 !important;
  background: none !important;
  pointer-events: none;
  z-index: 1;
}
findbar > .findbar-container,
findbar > .close-icon {
  background-color: var(--lwt-accent-color, var(--toolbox-bgcolor)) !important;
  pointer-events: auto;
  border-bottom-right-radius: 4px;
}
findbar[hidden] {
  transform: translateY(-30px);
}

Source: firefox-csshacks/chrome/floating_findbar_on_top.css

Overlay fullscreen toolbars

Toolbars float over content in fullscreen instead of pushing it down:

@media -moz-pref("browser.fullscreen.autohide") {
  :root[sizemode="fullscreen"] #navigator-toolbox {
    position: fixed !important;
    width: 100vw;
    z-index: 10 !important;
    transition: transform 82ms ease-in-out 600ms !important;
    transform: translateY(-100%);
  }
  :root[sizemode="fullscreen"] #navigator-toolbox:is(:hover, :focus-within) {
    transition-delay: 0ms !important;
    transform: translateY(0);
  }
}

Source: firefox-csshacks/chrome/overlay_fullscreen_toolbars.css

Compact proton

Variables for compact density:

:root {
  --toolbarbutton-inner-padding: 6px !important;
  --tab-block-margin: 2px !important;
  --tabs-shadow-size: 0px !important;
  --arrowpanel-menuitem-padding-block: 5px !important;
  --panel-font-size: inherit !important;
  --arrowpanel-padding: 0.8em !important;
  --tab-inline-padding: 8px !important;
}
#nav-bar {
  box-shadow: inset 0 var(--tabs-shadow-size) 0 var(--lwt-tabs-border-color) !important;
}
.tab-close-button {
  width: 20px !important;
  height: 20px !important;
  padding: 5px !important;
}

Source: firefox-csshacks/chrome/compact_proton.css

Compact urlbar

Prevents urlbar breakout expansion:

#urlbar[breakout][breakout-extend] {
  margin-left: 0 !important;
  width: var(--urlbar-width) !important;
  margin-top: calc(
    (var(--urlbar-container-height) - var(--urlbar-height)) / 2
  ) !important;
}
:where(#urlbar) > .urlbar-background,
#urlbar-background {
  animation: none !important;
}
:where(#urlbar) > .urlbar-input-container {
  padding: var(--urlbar-container-padding, 0) 1px !important;
  height: var(--urlbar-height) !important;
}

Source: firefox-csshacks/chrome/compact_urlbar_megabar.css

Rounded menupopups

Rounding panels, menus, urlbar consistently:

:root {
  --uc-menupopup-border-radius: 20px;
}
panel[type="autocomplete-richlistbox"],
menupopup,
.panel-arrowcontent {
  -moz-appearance: none !important;
  border-radius: var(--uc-menupopup-border-radius) !important;
  overflow: clip !important;
}
/* Match urlbar and searchbar */
searchbar#searchbar,
:where(#urlbar) > .urlbar-background,
#urlbar-background {
  border-radius: var(--uc-menupopup-border-radius) !important;
}
/* Fix panel arrow position */
panel[type="arrow"] {
  margin-inline-end: calc(-10px - var(--uc-menupopup-border-radius)) !important;
}
.panel-arrow {
  margin-inline: var(--uc-menupopup-border-radius) !important;
}

Source: firefox-csshacks/chrome/rounded_menupopups.css

Minimal toolbar buttons

Hide buttons as dots, reveal on hover using transform: scale(0):

toolbar .toolbarbutton-1 > * {
  transform: scale(0);
  transition: transform 82ms linear !important;
}
toolbar:hover .toolbarbutton-1 > * {
  transform: scale(1);
}
/* Dot placeholder */
toolbar .toolbarbutton-1:not([open]) {
  background-image: radial-gradient(
    circle at center,
    currentColor 0,
    currentColor 10%,
    transparent 15%
  );
}
toolbar:hover .toolbarbutton-1 {
  background-image: none;
}

Source: firefox-csshacks/chrome/minimal_toolbarbuttons_v3.css

Color variable template

Key variables for full theme override:

:root {
  --arrowpanel-background: <color> !important;
  --arrowpanel-border-color: <color> !important;
  --arrowpanel-color: <color> !important;
  --lwt-accent-color: <color> !important;
  --toolbar-bgcolor: <color> !important;
  --tab-selected-bgcolor: <color> !important;
  --lwt-text-color: <color> !important;
  --toolbarbutton-icon-fill: <color> !important;
  --toolbar-field-background-color: <color> !important;
  --toolbar-field-focus-background-color: <color> !important;
  --toolbar-field-color: <color> !important;
  --toolbar-field-focus-color: <color> !important;
  --toolbar-field-border-color: <color> !important;
  --toolbar-field-focus-border-color: <color> !important;
  --lwt-sidebar-background-color: <color> !important;
  --lwt-sidebar-text-color: <color> !important;
}
#navigator-toolbox {
  --lwt-tabs-border-color: <color> !important;
}
#tabbrowser-tabs {
  --lwt-tab-line-color: <color> !important;
}
#sidebar-box {
  --sidebar-background-color: <color> !important;
}

Source: firefox-csshacks/chrome/color_variable_template.css

Linux GTK window controls patch

CSD buttons don't respect layout rules by default on Linux:

.titlebar-buttonbox {
  align-items: stretch !important;
}
.titlebar-button {
  -moz-appearance: none !important;
  -moz-context-properties: fill, stroke, fill-opacity;
  fill: currentColor;
  padding: 4px 6px !important;
  flex-grow: 1;
  overflow: clip;
}

Source: firefox-csshacks/chrome/linux_gtk_window_control_patch.css

Window controls on left

Use @media -moz-pref("userchrome.force-window-controls-on-left.enabled"):

@media -moz-pref("userchrome.force-window-controls-on-left.enabled") {
  #nav-bar > .titlebar-buttonbox-container {
    order: -1 !important;
    > .titlebar-buttonbox {
      flex-direction: row-reverse;
    }
  }
}

Also detectable via (-moz-gtk-csd-reversed-placement) or (-moz-platform: macos).

-moz-pref() media queries

Firefox 133+ supports @media -moz-pref("pref.name") for conditional CSS based on about:config prefs. Used extensively by firefox-csshacks for toggleable features:

• sidebar.verticalTabs — native vertical tabs enabled
• browser.fullscreen.autohide — fullscreen toolbar autohide
• userchrome.* — custom user prefs for CSS feature toggles

Common mistakes

• #id vs .class — check source (e.g. .private-browsing-indicator-with-label is a class, not an ID)
• Shadow DOM parts (scrollbutton-up, scrollbutton-down) are ::part(), not IDs
• tab bare element doesn't match — use .tabbrowser-tab
• Pseudo-elements (::before, ::after) — verify they exist in source before targeting
• Border/separator overrides — find which element has the border (e.g. .urlbarView-body-inner, not .urlbarView-body-outer)
• .searchbar-engine-one-off-item is not inside #urlbar
• #context-sep-navigation is in browser.xhtml, not in CSS
• #pocket-button, #scrollbutton-up, #scrollbutton-down do not exist in browser.xhtml
• order / -moz-box-ordinal-group for layout reordering — use CSS grid instead
• Setting CSS variables without !important direct overrides — internal styles may not consume the variable
• Relying on visibility: collapse without zeroing min-height on all child containers
• Missing contain: strict on periphery elements when collapsing toolbars — they hold height open
• Adding !important to variables — variables are automatically applied, !important on them is pointless
• Duplicate user_pref() lines in user.js — last one wins silently, remove duplicates
• GNOME theme prefs (gnomeTheme.*) conflict with custom tab positioning