Occasionally, you’ll want to use Firefox in another language instead of the one you’re speaking currently. How do you achieve that without reinstalling Firefox? I’m trying to answer this question and talk about related concepts such as locale and language packs.
When you encounter this problem and you google around, you’ll probably notice a Firefox preference general.useragent.locale quickly. Then you change the value of this parameter as what is told, but nothing happens when you restart Firefox.
Why’s that? To find out the reason, we have to dig deeper into the concept user agent and locale. The user agent string is a piece of text which identifies the name and version of a given browser. It is used by websites to determine which browser you use. That is to say, the user agent is your browser’s identity. The current locale in use by Mozilla is stored in the preference general.useragent.locale, and a variety of components consult it for localization information. Remember when you visit YouTube for the first time and the page prompts you for the language you are using intelligently? Oh yeah, that’s the magic generated by our user agent string. You get it now, simply altering this preference just changes how your browser identifies itself, not necessarily its appearance.
Hence what you need essentially to get a different UI language is the language pack, besides changing this preference. A language pack is an extension that changes the language of the user interface in a Mozilla application. Alternatively, you can assign a different user-interface language to each command or icon that you use to start the Mozilla application. For example, to use French you can specify either: -uilocale fr or -uilocale fr-FR. On a Windows system the entire command might look something like:
C:Program FilesMozillaThunderbirdthunderbird.exe -uilocale fr
To download the language packs, visit
In my case, that is