Cross-Browser Extension Compatibility

Comprehensive guide to writing browser extensions that work across Chrome, Firefox, Safari, and Edge with proper feature detection and polyfills.

Overview

Browser extensions share a common WebExtensions API standard, but implementations differ significantly. This skill covers how to handle those differences.

This skill covers:

API compatibility matrices
Polyfill usage and patterns
Feature detection techniques
Browser-specific workarounds
Manifest differences

This skill does NOT cover:

General JavaScript compatibility (use caniuse.com)
Extension store submission (see extension-anti-patterns skill)
UI framework differences

Quick Reference

Browser API Namespaces

Browser	Namespace	Promises	Polyfill Needed
Chrome	`chrome.*`	Callbacks	Yes
Firefox	`browser.*`	Native	No
Safari	`browser.*`	Native	No
Edge	`chrome.*`	Callbacks	Yes

Universal Pattern

// Use webextension-polyfill for consistent API
import browser from 'webextension-polyfill';

// Now works in all browsers with Promises
const tabs = await browser.tabs.query({ active: true });

API Compatibility Matrix

Core APIs

API	Chrome	Firefox	Safari	Edge	Notes
`action.*`	✓	✓	✓	✓	MV3 only
`alarms.*`	✓	✓	✓	✓	Standard
`bookmarks.*`	✓	✓	✗	✓	Safari: no support
`browserAction.*`	MV2	✓	MV2	MV2	Use `action` in MV3
`commands.*`	✓	✓	◐	✓	Safari: limited
`contextMenus.*`	✓	✓	✓	✓	Standard
`cookies.*`	✓	✓	◐	✓	Safari: restrictions
`downloads.*`	✓	✓	✗	✓	Safari: no support
`history.*`	✓	✓	✗	✓	Safari: no support
`i18n.*`	✓	✓	✓	✓	Standard
`identity.*`	✓	◐	✗	✓	Firefox: partial
`idle.*`	✓	✓	✗	✓	Safari: no support
`management.*`	✓	✓	✗	✓	Safari: no support
`notifications.*`	✓	✓	✗	✓	Safari: no support
`permissions.*`	✓	✓	◐	✓	Safari: limited
`runtime.*`	✓	✓	✓	✓	Standard
`scripting.*`	✓	✓	◐	✓	Safari: limited
`storage.*`	✓	✓	✓	✓	Standard
`tabs.*`	✓	✓	◐	✓	Safari: some limits
`webNavigation.*`	✓	✓	◐	✓	Safari: limited
`webRequest.*`	✓	✓	◐	✓	Safari: observe only
`windows.*`	✓	✓	◐	✓	Safari: limited

Advanced APIs

API	Chrome	Firefox	Safari	Edge	Workaround
`declarativeNetRequest`	✓	◐	◐	✓	Use webRequest
`offscreen`	109+	✗	✗	109+	Content script
`sidePanel`	114+	✗	✗	114+	Use popup
`storage.session`	102+	115+	16.4+	102+	Use local + clear
`userScripts`	120+	✓	✗	120+	Content scripts

Polyfill Setup

Using webextension-polyfill

The Mozilla webextension-polyfill normalizes the Chrome callback-style API to Firefox's Promise-based API.

Installation

npm install webextension-polyfill
# TypeScript types
npm install -D @anthropic-ai/anthropic-sdk-types/webextension-polyfill

Usage in Background Script

// background.ts
import browser from 'webextension-polyfill';

browser.runtime.onMessage.addListener(async (message, sender) => {
  const tabs = await browser.tabs.query({ active: true, currentWindow: true });
  return { tabId: tabs[0]?.id };
});

Usage in Content Script

// content.ts
import browser from 'webextension-polyfill';

const response = await browser.runtime.sendMessage({ type: 'getData' });
console.log(response);

WXT Framework (Recommended)

WXT provides built-in polyfill support:

// No import needed - browser is global
export default defineContentScript({
  matches: ['*://*.example.com/*'],
  main() {
    // browser.* works everywhere
    browser.runtime.sendMessage({ type: 'init' });
  },
});

Feature Detection Patterns

Check API Availability

// Check if API exists
function hasAPI(api: string): boolean {
  const parts = api.split('.');
  let obj: any = typeof browser !== 'undefined' ? browser : chrome;

  for (const part of parts) {
    if (obj && typeof obj[part] !== 'undefined') {
      obj = obj[part];
    } else {
      return false;
    }
  }
  return true;
}

// Usage
if (hasAPI('sidePanel.open')) {
  browser.sidePanel.open({ windowId });
} else {
  // Fallback to popup
  browser.action.openPopup();
}

Runtime Browser Detection

// Detect browser at runtime
function getBrowser(): 'chrome' | 'firefox' | 'safari' | 'edge' | 'unknown' {
  const ua = navigator.userAgent;

  if (ua.includes('Firefox')) return 'firefox';
  if (ua.includes('Safari') && !ua.includes('Chrome')) return 'safari';
  if (ua.includes('Edg/')) return 'edge';
  if (ua.includes('Chrome')) return 'chrome';

  return 'unknown';
}

// Detect from extension APIs
function getBrowserFromAPIs(): 'chrome' | 'firefox' | 'safari' | 'edge' {
  if (typeof browser !== 'undefined') {
    // @anthropic-ai/anthropic-sdk-ts-expect-error - browser_specific_settings only in Firefox
    if (browser.runtime.getBrowserInfo) return 'firefox';
    return 'safari';
  }
  if (navigator.userAgent.includes('Edg/')) return 'edge';
  return 'chrome';
}

Feature Flags Pattern

// features.ts
export const FEATURES = {
  sidePanel: hasAPI('sidePanel'),
  offscreen: hasAPI('offscreen'),
  sessionStorage: hasAPI('storage.session'),
  userScripts: hasAPI('userScripts'),
  declarativeNetRequest: hasAPI('declarativeNetRequest'),
} as const;

// Usage
import { FEATURES } from './features';

if (FEATURES.sidePanel) {
  // Use side panel
} else {
  // Use popup alternative
}

Browser-Specific Patterns

Firefox-Specific

Gecko ID (Required)

{
  "browser_specific_settings": {
    "gecko": {
      "id": "my-extension@example.com",
      "strict_min_version": "109.0"
    }
  }
}

Data Collection Permissions (2025+)

{
  "browser_specific_settings": {
    "gecko": {
      "id": "my-extension@example.com",
      "data_collection_permissions": {
        "required": [],
        "optional": ["technicalAndInteraction"]
      }
    }
  }
}

Firefox Android Support

{
  "browser_specific_settings": {
    "gecko": {
      "id": "my-extension@example.com"
    },
    "gecko_android": {
      "strict_min_version": "120.0"
    }
  }
}

Safari-Specific

Privacy Manifest Requirement

Safari extensions require a host app with PrivacyInfo.xcprivacy:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "...">
<plist version="1.0">
<dict>
  <key>NSPrivacyTracking</key>
  <false/>
  <key>NSPrivacyCollectedDataTypes</key>
  <array/>
</dict>
</plist>

Safari Limitations Handling

// Safari doesn't support webRequest blocking
async function blockRequest(details: WebRequestDetails) {
  const browser = getBrowser();

  if (browser === 'safari') {
    // Use declarativeNetRequest instead
    await browser.declarativeNetRequest.updateDynamicRules({
      addRules: [{
        id: 1,
        action: { type: 'block' },
        condition: { urlFilter: details.url }
      }]
    });
  } else {
    // Use webRequestBlocking
    return { cancel: true };
  }
}

Chrome-Specific

Service Worker State Persistence

// Chrome service workers terminate after ~5 minutes
// Always persist state to storage

// BAD: State lost on worker termination
let count = 0;

// GOOD: Persist to storage
const countStorage = storage.defineItem<number>('local:count', {
  defaultValue: 0
});

async function increment() {
  const count = await countStorage.getValue();
  await countStorage.setValue(count + 1);
}

Offscreen Documents (Chrome/Edge only)

// For DOM access in MV3 service worker
if (hasAPI('offscreen')) {
  await chrome.offscreen.createDocument({
    url: 'offscreen.html',
    reasons: ['DOM_PARSER'],
    justification: 'Parse HTML content'
  });
}

Manifest Differences

Cross-Browser Manifest Generation

// wxt.config.ts
export default defineConfig({
  manifest: ({ browser }) => ({
    name: 'My Extension',
    version: '1.0.0',

    // Chrome/Edge
    ...(browser === 'chrome' && {
      minimum_chrome_version: '116',
    }),

    // Firefox
    ...(browser === 'firefox' && {
      browser_specific_settings: {
        gecko: {
          id: 'my-extension@example.com',
          strict_min_version: '109.0',
        },
      },
    }),

    // Different permissions per browser
    permissions: [
      'storage',
      'activeTab',
      ...(browser !== 'safari' ? ['notifications'] : []),
    ],
  }),
});

MV2 vs MV3 Differences

Feature	MV2	MV3
Background	`background.scripts`	`background.service_worker`
Remote code	Allowed	Forbidden
`executeScript`	Eval strings allowed	Functions only
Content security	Relaxed CSP	Strict CSP
`webRequestBlocking`	Supported	Use DNR

Testing Cross-Browser

Manual Testing Matrix

| Feature | Chrome | Firefox | Safari | Edge | Notes |
|---------|--------|---------|--------|------|-------|
| Install | [ ] | [ ] | [ ] | [ ] | |
| Popup opens | [ ] | [ ] | [ ] | [ ] | |
| Content script | [ ] | [ ] | [ ] | [ ] | |
| Background messages | [ ] | [ ] | [ ] | [ ] | |
| Storage sync | [ ] | [ ] | [ ] | [ ] | |

Automated Testing

// tests/browser-compat.test.ts
import { describe, it, expect } from 'vitest';
import { fakeBrowser } from 'wxt/testing';

describe('cross-browser compatibility', () => {
  it('handles missing sidePanel API', async () => {
    // Simulate Safari (no sidePanel)
    delete (fakeBrowser as any).sidePanel;

    const result = await openUI();
    expect(result.method).toBe('popup');
  });

  it('handles missing notifications API', async () => {
    delete (fakeBrowser as any).notifications;

    const result = await notify('Test');
    expect(result.fallback).toBe('console');
  });
});

Common Compatibility Issues

Issue: tabs.query Returns Different Results

Problem: Safari returns fewer tab properties.

Solution:

const tabs = await browser.tabs.query({ active: true });
const tab = tabs[0];

// Always check property existence
const url = tab?.url ?? 'unknown';
const favIconUrl = tab?.favIconUrl ?? '/default-icon.png';

Issue: Storage Quota Differences

Browser	Local	Sync	Session
Chrome	10MB	100KB	10MB
Firefox	Unlimited	100KB	10MB
Safari	10MB	100KB	10MB