This documentation aims at giving an overview of the preferences API used in DevTools, it is not an actual documentation about the list of preferences available in DevTools.


Preferences allows you to save and read strings, numbers, booleans to the preferences store, which is tied to a profile. A preference can also have a default value.

The technical solution for handling preferences differs depending whether you are testing DevTools as Firefox panel, or a standalone tool running with Launchpad.

Preference types

DevTools relies on nsIPrefBranch for preferences, which supports different types of preferences:

  • Int
  • Boolean
  • Char
  • String

Choose the appropriate type depending on the data you need to store. If you need to store a JavaSript object or array, the recommended way is to:

  • use a String type preference
  • use JSON.stringify to save
  • use JSON.parse to read

Note that nsIPrefBranch also supports a Complex type, but this type is not supported when running in Launchpad.

Reading and updating preferences

API docs for nsIPrefBranch and nsIPrefService

DevTools relies on Services.pref to handle preferences. You can access the API docs for this service at:

If you are using Launchpad, note that only a subset of nsIPrefService methods are implemented (addObserver and removeObserver). Launchpad relies on a Services shim file provided by devtools-module (code on GitHub).

Requiring Services.pref

To require Services and use Services.pref, you can normally use the following snippet:

const Services = require("Services");

In the rare event where you don't have access to the DevTools' require method, you can use

const { Services } = Components.utils.import("resource://gre/modules/Services.jsm", {});

Services.pref.get and Services.pref.set

The main APIs you will have to know and use are getters and setters.

  • Services.pref.getIntPref(prefName, defaultValue); This method will throw if the preference cannot be found and you didn't pass a default value!
  • Services.pref.setIntPref(prefName, prefValue) This method will throw if the provided value does not match the preference type!

These APIs are very similar for each preference type.

Create a new preference

To create a new preference, it should be assigned a default value. Default preferences are defined in preferences files such as:

  • devtools/client/preferences/devtools.js
  • devtools/client/preferences/debugger.js
  • devtools/shim/devtools-startup-prefs.js

Most new preferences should go in devtools/client/preferences/devtools.js. Debugger specific preferences should go in devtools/client/preferences/debugger.js. Finally if a preference needs to be available very early during the Firefox startup sequence, it should go in devtools/shim/devtools-startup-prefs.js.

Projects using Launchpad

At the time of writing this doc, projects using Launchpad have to duplicate the default definition of a preference.

Inspect preferences

Depending on the project you are working on, preferences are stored differently but can always be inspected.

In Firefox, you can open a tab to about:config and search by preference name.

In Launchpad, preferences are actually saved to localStorage. Open DevTools on your Launchpad application and inspect the local storage content. You should see entries prefixed by Services.prefs:. You will only see preferences where a user-specific value has overridden the default value.

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