Firefox browser add-on development

Firefox Add-on Development

Firefox Add-ons are based on WebExtensions API — the same specification as Chrome Extensions. Most Chrome extensions can be ported to Firefox with minimal changes. However, there are differences in details: API support, signing procedure, extension engine.

Firefox vs Chrome differences

Manifest V2 (Firefox — most stable variant)

{
  "manifest_version": 2,
  "name": "My Firefox Add-on",
  "version": "1.0.0",
  "description": "Add-on description",
  "permissions": [
    "storage",
    "tabs",
    "activeTab",
    "https://*.example.com/*"
  ],
  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icons/icon48.png",
      "96": "icons/icon96.png"
    }
  },
  "content_scripts": [{
    "matches": ["https://*.target-site.com/*"],
    "js": ["content.js"]
  }],
  "options_ui": {
    "page": "options.html",
    "open_in_tab": false
  }
}

Promise-based API

Firefox supports native Promises in browser.*. The webextension-polyfill polyfill makes code compatible with Chrome:

import browser from 'webextension-polyfill';

// Firefox — native promises
const tabs = await browser.tabs.query({ active: true, currentWindow: true });
const tab = tabs[0];

// Execute script (MV2)
await browser.tabs.executeScript(tab.id, {
  code: 'document.body.style.background = "yellow"',
});

// Storage
await browser.storage.local.set({ key: 'value' });
const result = await browser.storage.local.get('key');

Content Script: limitations and workarounds

In Firefox, Content Scripts run in an isolated world (Xray wrapper) — they don't have direct access to JavaScript objects on the page. For interaction with the page, you need window.postMessage or CustomEvent:

// Content script -> Page script
window.postMessage({ source: 'myExtension', type: 'REQUEST_DATA' }, '*');

// Page script handler
window.addEventListener('message', (e) => {
  if (e.data?.source === 'myExtension' && e.data?.type === 'REQUEST_DATA') {
    window.postMessage({
      source: 'myPage',
      type: 'RESPONSE_DATA',
      payload: window.__MY_APP_STATE__,
    }, '*');
  }
});

Signing and publishing

Firefox requires signing from Mozilla for any extension not distributed through AMO:

# web-ext — official CLI from Mozilla
npm install -g web-ext

# Development with hot reload
web-ext run --source-dir ./dist --firefox-binary "/path/to/firefox"

# Build
web-ext build --source-dir ./dist --artifacts-dir ./artifacts

# Signing (requires AMO API keys)
web-ext sign \
  --source-dir ./dist \
  --api-key $AMO_JWT_ISSUER \
  --api-secret $AMO_JWT_SECRET

To get API keys: addons.mozilla.org/developers/addon/api/key/.

Temporary installation for testing

Without signing, the add-on can only be installed via about:debugging:

1. Open about:debugging#/runtime/this-firefox
2. "Load Temporary Add-on"
3. Select manifest.json
4. Works until browser restart

For corporate deployment without AMO — use policies.json in Firefox Enterprise.

Firefox specifics: contextualIdentities (containers)

Firefox supports Containers — isolated contexts with separate cookies. Access via API:

const containers = await browser.contextualIdentities.query({});
// { cookieStoreId, name, color, icon }

// Open tab in specific container
await browser.tabs.create({
  url: 'https://example.com',
  cookieStoreId: 'firefox-container-1',
});

Timeline

Firefox Add-on adapted from Chrome extension, with signing and AMO publication — 2–3 additional days on top of ready Chrome extension. New add-on development from scratch — 4–8 days depending on functionality.