Autoload Temporary Firefox Addons (ESM-Update for Firefox 136+)

Fork of tsaost/autoload-temporary-addon — updated for modern Firefox versions.

Problem

Firefox does not allow permanently installing unsigned, self-developed extensions. The only way is to load them as "temporary add-ons" via about:debugging, but they are lost on every restart.

The original project solved this using Firefox AutoConfig (userChrome.js), but stopped working with Firefox 136+ because Mozilla removed the legacy JSM module system (Services.jsm, AddonManager.jsm, FileUtils.jsm).

What changed

This fork replaces the old JSM imports with the new ESM equivalents:

// Old (broken in Firefox 136+):
Components.utils.import("resource://gre/modules/Services.jsm");
Components.utils.import("resource://gre/modules/AddonManager.jsm");
Components.utils.import("resource://gre/modules/FileUtils.jsm");

// New (works in Firefox 136+):
var Services = globalThis.Services;
var { AddonManager } = ChromeUtils.importESModule("resource://gre/modules/AddonManager.sys.mjs");
var { FileUtils } = ChromeUtils.importESModule("resource://gre/modules/FileUtils.sys.mjs");

Tested on Firefox 148 (Linux).

Installation

Step 1: Copy config-prefs.js to Firefox defaults

# Linux:
sudo cp config-prefs.js /usr/lib/firefox/defaults/pref/

# macOS:
sudo cp config-prefs.js /Applications/Firefox.app/Contents/Resources/defaults/pref/

# Windows (as admin):
copy config-prefs.js "C:\Program Files\Mozilla Firefox\defaults\pref\"

Step 2: Edit userChrome.js

Open userChrome.js and add your extension paths to installUnpackedExtensions():

function installUnpackedExtensions() {
    installExtension("/home/user/my-extension");
    installExtension("/home/user/another-extension");
}

Step 3: Copy userChrome.js to Firefox install directory

# Linux:
sudo cp userChrome.js /usr/lib/firefox/

# macOS:
sudo cp userChrome.js /Applications/Firefox.app/Contents/Resources/

# Windows (as admin):
copy userChrome.js "C:\Program Files\Mozilla Firefox\"

Step 4: Restart Firefox

Your extensions will be automatically loaded at startup.

Important

• Your extensions must have a browser_specific_settings entry in manifest.json with a unique gecko.id, otherwise local storage will be lost on restart:

  "browser_specific_settings": {
    "gecko": { "id": "your-extension@example.com" }
  }

• This is functionally identical to clicking "Load Temporary Add-on" in about:debugging — it just happens automatically.
• The extensions require files in the Firefox install directory (/usr/lib/firefox/), which needs admin/root access. This is not a new security concern — if an attacker has write access to that directory, they can already modify firefox itself.

Why is this needed?

Mozilla does not provide a "Developer Mode" like Chrome. The only alternatives are:

• Sign extensions through Mozilla (requires account + internet)
• Use Firefox ESR or Developer Edition (can disable signature checks)
• Load via about:debugging every time (lost on restart)

See Bug 1309288 for the long-standing feature request.

Credits

• Original project by tsaost
• ESM update by Mantik IT