In an enterprise environment, it may be necessary to deploy Firefox with language packs preinstalled. This document will show you how to do that. These steps also work if you are customizing the Windows installer.

Step 1: Create directories

In the Firefox installation directory, you will need to create some required directories. First, create a directory called distribution. Inside of that directory, create another called extensions. If you are on macOS, you should create the directories in Contents/Resources

Step 2: Download language packs

Download the language packs that correspond to the build you are installing. They are in the xpi directory for the corresponding platform. For example:

https://releases.mozilla.org/pub/firefox/releases/89.0/win64/xpi/

Even though the XPI files are duplicated in the different platforms, there is no difference between them. You can download the XPI from any platform directory.

Step 3: Rename the language packs

Rename the language packs using the following format:

xx.xpi -> langpack-xx@firefox.mozilla.org.xpi

xx-XX.xpi -> langpack-xx-XX@firefox.mozilla.org.xpi

Step 4 : Copy the language packs

Copy the renamed language packs into the extensions directory you created in step 1.

Step 5: Set the default language

Using policy, set RequestedLocales to either the language you want to use by default or an empty string to use the same language as the operating system.