Creating addons for SlimerJSΒΆ

Since SlimerJS is using Firefox’s framework, it is theorically possible to create and install XUL addons for SlimerJS. But it works only if you are using Firefox 56 or lower. The support of XUL addons has been removed in Firefox 57+. And the support of WebExtensions has not been yet tested with SlimerJS.

With Firefox 56-, even it is possible, it’s not so easy to install XUL addons:

  • Their installation is not easy since their are no user interface to install them.

  • An addon made for Firefox may not work with SlimerJS because :
    • the user interface is not the same, so the addon may not found what it wants. For example, there is no toolbar to add a button, so the extension code may not work when it wants to add a button.
    • the install.rdf manifest of an addon may only refer to Firefox as application target. The addon should at least refers to the mozilla toolkit (see below).

However, this is not impossible. Here are some tips.

Warning: to understand following instructions, you need to learn how to create addons for Firefox first. See documentation on MDN.

Steps to install extensions:

    1. set targetApplication to (install.rdf)

    SlimerJS doesn’t run extensions dedicated to firefox. Your extension must either be dedicated to targetApplication or in case of a hybrid extension more generically to


    1. EnableExtensionManager=1 (application.ini)

    In order to convince SlimerJS to run extension, an addition to its application.ini is required.


    1. Set User Prefs enabledAddons (prefs.js)

    Add-ons need to be enabled. As a UI is missing to enable the add-on, you need to set the configuration manually. This is done in the profile dir prefs.js file. The preference contains a comma-separated list of Extension_IDs + their version. This must fit to <em:id>...</em:id> and <em:version>...</em:version> of the extensions install.rdf (see step 1). Remember to run SlimerJS with your profile: -profile ....


    user_pref("extensions.enabledAddons", "{EXTENSION_ID}:{VERSION},..");
    1. Set User Prefs extensions.xpiState (prefs.js)

    Add-ons need to be loaded. As a UI is missing to load an extension, you need to set user preferences manually. This is done in the profile dir prefs.js file. The value is a JSON object (be careful with masking quots and structure). You need to set EXTENSION_ID, realFolderToExtension and VERSION accordingly. The timestamps fields (st and mt) should be filled with recent timestamps. Everything in the near past should do. Remember to run SlimerJS with your profile: -profile ....


    user_pref("extensions.xpiState", "{\"app-profile\":{\"#EXTENSION_ID#\":{\"d\":\"#realFolderToExtension#\",\"e\":true,\"v\":\"#VERSION#\",\"st\":1450636700000,\"mt\":1447437723000}}}");
    1. Set correct overlay (chrome.manifest)

    As SlimerJS doesn’t come with browser.xul, your extension need to overlay the slimerjs.xul or webpage.xul instead, if your extension uses overlays at all.


    overlay   chrome://slimerjs/content/webpage.xul chrome://path/to/your/overlay.xul
    Note: SlimerJS is supposed to not have a user interface. Avoid to use
    overlays to add some user interface components, since in a "ghost" context,
    (like running tests on the CI plateform...), nobody will click on buttons
    of the user interface ;)

Also see documentation on MDN.

One additional hint: Many Firefox extensions are using Fuel. SlimerJS doesn’t include Fuel and this application Interface is deprecated anyway. Use SDK components instead. The following example shows both versions for accessing extension information.


var app = Cc[";1"].getService(Ci.fuelIApplication);
app.getExtensions(function (extensions) {
  constants.VERSION = extensions.get(constants.EXTENSION_ID).version;


AddonManager.getAddonByID(constants.EXTENSION_ID, function(addon) {
  constants.VERSION = addon.version;

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