Firefox WebExtension Development Skill

Firefox WebExtension Development

Complete reference for building, testing, and publishing browser extensions for Mozilla Firefox.

Overview

Firefox extensions use the WebExtensions API with the browser.* namespace (Promise-based natively). Firefox supports both Manifest V2 and V3, with MV2 NOT deprecated (unlike Chrome).

Key Characteristics:

Global namespace: browser (Promise-based)
Both MV2 and MV3 supported
Firefox-specific APIs: sidebarAction, userScripts, contextualIdentities, protocol_handlers
Submission via AMO (addons.mozilla.org)

Manifest Structure

Manifest V3 (Recommended, Firefox 109+)

{
  "manifest_version": 3,
  "name": "Extension Name",
  "version": "1.0.0",
  "description": "Brief description",

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

  "background": {
    "service_worker": "background.js",
    "type": "module"
  },

  "action": {
    "default_popup": "popup.html",
    "default_icon": {
      "16": "icons/icon-16.png",
      "48": "icons/icon-48.png",
      "96": "icons/icon-96.png"
    },
    "default_title": "Extension Title"
  },

  "content_scripts": [
    {
      "matches": ["https://*/*"],
      "js": ["content.js"],
      "css": ["styles.css"],
      "run_at": "document_idle"
    }
  ],

  "permissions": ["storage", "activeTab"],
  "host_permissions": ["https://api.example.com/*"],

  "content_security_policy": {
    "extension_pages": "script-src 'self'; object-src 'self';"
  }
}

> See `references/security-crossbrowser.md` for full CSP reference.

### Manifest V2 (Still Supported)

```json
{
  "manifest_version": 2,
  "name": "Extension Name",
  "version": "1.0.0",

  "browser_specific_settings": {
    "gecko": {
      "id": "extension@example.com",
      "strict_min_version": "78.0"
    }
  },

  "background": {
    "scripts": ["background.js"],
    "persistent": false
  },

  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icons/icon-48.png"
  },

  "permissions": ["storage", "activeTab", "https://*/*"]
}

MV2 vs MV3 Key Differences

| Feature | MV2 | MV3 | Firefox Version Notes | |---------|-----|-----|----------------------| | Toolbar button | browser_action | action | action available since Firefox 109+ | | Background | background.scripts / page | background.service_worker | Service workers since Firefox 109+; MV2 background pages still supported | | Host permissions | In permissions | Separate host_permissions | MV3 separation enforced from Firefox 109+ | | Default CSP | script-src 'self'; object-src 'self'; | script-src 'self'; upgrade-insecure-requests; | CSP tightening in MV3 blocks inline scripts | | Request blocking | webRequest.onBeforeRequest | declarativeNetRequest | MV3: webRequest is non-blocking only; blocking requires declarativeNetRequest (rule limits apply) | | MV3 availability | N/A | Supported | Firefox 109+ (ESR 115+) — verify against MDN for ESR boundaries | | MV2 deprecation | Active | N/A | Firefox has NOT deprecated MV2 (unlike Chrome); verify current status on MDN |

MV3 forces these practical changes:

No remote code: all logic must be bundled; no eval(), no dynamic import() from network
background.scripts → background.service_worker (event-driven, ephemeral)
CSP tightening: inline scripts (onclick="...") blocked; move to event listeners
webRequest blocking → declarativeNetRequest (max 30,000 rules, 5,000 dynamic rules)

All Manifest Keys Reference

Metadata:

name (required) - Extension name
version (required) - Version string (e.g., "1.0.0")
description - Short description
author - Author name
homepage_url - Extension homepage
icons - Extension icons object

Firefox-Specific:

browser_specific_settings.gecko.id - Required for AMO
browser_specific_settings.gecko.strict_min_version - Minimum Firefox version
browser_specific_settings.gecko.strict_max_version - Maximum Firefox version
browser_specific_settings.gecko_android - Android-specific settings

Background & Scripts:

background - Service worker (MV3) or scripts/page (MV2)
content_scripts - Scripts injected into pages
userScripts - User script registration (Firefox-only)
declarative_net_request - Rule-based request modification

UI Components:

action (MV3) / browser_action (MV2) - Toolbar button
page_action - Address bar button
sidebar_action - Sidebar panel (Firefox-only)
options_ui - Options page configuration
devtools_page - DevTools extension page

Permissions:

permissions - API permissions
host_permissions (MV3) - Host access
optional_permissions - Optional API permissions
optional_host_permissions - Optional host access

Permissions Justification Discipline

High-risk permissions trigger AMO reviewer scrutiny. Document the narrowest alternative:

| Permission | What it grants | Narrower alternative | |------------|----------------|---------------------| | <all_urls> | Access to every website | Specific host patterns (https://*.example.com/*) | | webRequest | Inspect/modify all network traffic | declarativeNetRequest for static rules; limit scope with host_permissions | | cookies | Read/write all cookies | host_permissions for specific domains; justify in AMO submission | | debugger | Attach debugger to any tab | Rarely justified; consider devtools API instead | | tabs | Read all tab metadata | activeTab for on-demand access; scripting API for content injection | | bookmarks | Read/modify all bookmarks | None — justify necessity in AMO notes | | history | Read browsing history | None — AMO requires strong justification |

AMO review trigger: Permission justification must appear in the extension listing description or AMO notes. Vague justifications lead to rejection.

Other:

commands - Keyboard shortcuts
omnibox - Address bar integration
web_accessible_resources - Resources accessible from pages
protocol_handlers - Custom protocol handlers
chrome_settings_overrides - Override homepage/search
chrome_url_overrides - Override new tab/bookmarks

web-ext CLI Reference

Installation

npm install -g web-ext

Commands

Run extension (development):

web-ext run                                    # Default Firefox
web-ext run --firefox-path /path/to/firefox    # Specific Firefox
web-ext run --target firefox-android           # Android
web-ext run --profile-create-new               # Clean profile
web-ext run --start-url https://example.com    # Start URL
web-ext run --verbose                          # Verbose output
web-ext run --no-reload                        # Disable auto-reload

Lint extension:

web-ext lint                   # Validate manifest and code
web-ext lint --warnings-as-errors
web-ext lint --self-hosted     # Skip AMO-specific checks

Build extension:

web-ext build                  # Create .zip in web-ext-artifacts/
web-ext build --source-dir ./src
web-ext build --artifacts-dir ./dist

Sign extension:

web-ext sign --api-key $KEY --api-secret $SECRET
web-ext sign --channel listed      # Public on AMO
web-ext sign --channel unlisted    # Direct download

Other commands:

web-ext docs       # Open documentation
web-ext dump-config  # Show configuration

Configuration File (.web-ext-config.js)

module.exports = {
  sourceDir: './src',
  artifactsDir: './dist',

  run: {
    firefox: '/Applications/Firefox.app/Contents/MacOS/firefox',
    startUrl: 'https://example.com',
    pref: ['extensions.webextensions.debug=true']
  },

  build: {
    overwriteDest: true
  },

  sign: {
    apiKey: process.env.AMO_API_KEY,
    apiSecret: process.env.AMO_API_SECRET,
    channel: 'listed'
  }
};

Environment Variables

WEB_EXT_API_KEY=your_key
WEB_EXT_API_SECRET=your_secret
WEB_EXT_SOURCE_DIR=./src
WEB_EXT_ARTIFACTS_DIR=./dist
WEB_EXT_FIREFOX=/path/to/firefox

Best Practices

Background Service Worker (MV3)

// Keep service worker alive briefly
let keepAlive;

browser.runtime.onMessage.addListener((msg) => {
  clearTimeout(keepAlive);
  keepAlive = setTimeout(() => {}, 25000);
  return handleMessage(msg);
});

Error Handling

async function safeAsync(fn) {
  try {
    return await fn();
  } catch (error) {
    console.error('Error:', error);
    return { error: error.message };
  }
}

Performance

Use browser.tabs.query() with specific filters
Debounce frequent operations
Cache storage API results
Use alarms API instead of setInterval in background

File Structure Template

my-extension/
├── manifest.json
├── background.js
├── content.js
├── popup.html
├── popup.js
├── options.html
├── options.js
├── browser-polyfill.js
├── styles/
│   ├── popup.css
│   └── content.css
├── icons/
│   ├── icon-16.png
│   ├── icon-32.png
│   ├── icon-48.png
│   └── icon-96.png
├── _locales/
│   ├── en/
│   │   └── messages.json
│   └── it/
│       └── messages.json
├── .web-ext-config.js
├── package.json
└── README.md

AMO Submission & Review

Official references:

Automatic Rejection Triggers

These issues cause immediate automated or quick human rejection:

| Issue | Fix | |-------|-----|| | Missing browser_specific_settings.gecko.id | Add unique UUID or email-based ID | | Data collection disclosure missing | Add data_collection field in manifest or privacy policy URL | | Homepage/source URL 404s | Verify URLs before submission; use permanent links | | Icon fails size rules | Provide 48x48 and 96x96 PNGs; no transparency issues | | Description reads as keyword spam | Write natural description; avoid stuffed keywords | | Missing permission justification | Document permission usage in AMO notes |

Human Reviewer Flags

Reviewers manually check for these red flags:

Remote code execution: Any eval(), new Function(), dynamic script loading from network
Overbroad host permissions: <all_urls> without clear justification
Data collection beyond declared purpose: Privacy policy must match actual behavior
Hidden functionality: Code paths not disclosed in listing
Obfuscated code: Minification ok; obfuscation triggers source code request

Review Timeline & Response

Automated validation: 5-15 minutes
Human review (listed): 1-7 days
Human review (unlisted): Faster, often same-day

When rejected:

Read reviewer notes carefully (found in Developer Hub)
Fix the specific issue; don't resubmit without changes
Add clarifying notes if the rejection was a misunderstanding
Resubmit within 30 days to keep the review queue position

Data Collection Disclosure

If your extension collects/transmits user data, you MUST:

Add a privacy policy URL in the manifest (homepage_url or dedicated field)
Disclose what data is collected, how, and why
Implement user consent before collection
Declare data retention policy

See references/testing-amo.md for the full AMO checklist.

Deep Dives

Load these reference files on demand for detailed information:

WebExtension APIs (51 namespaces, core patterns, Firefox-specific): references/apis.md
Security & Cross-Browser (CSP, webextension-polyfill): references/security-crossbrowser.md
Testing & AMO (debugging, troubleshooting, submission): references/testing-amo.md

Agent Skills: Firefox WebExtension Development

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