You might have an English only Firefox installed within your system and the question then occurs: How to change the language? The notes presented below should work somewhat universally among different systems using Firefox.
Mozilla.org has a website from where you can actually download different languages to localize your browser: https://addons.mozilla.org/en-US/firefox/language-tools/
The downside of that page is that when you are running Linux/Unix you might get some annoying errors telling you about browser’s incompatibility without any valid reason. To fix incompatibility issues, go and get this Firefox add-on: https://addons.mozilla.org/en-US/firefox/addon/language-pack-install-helpe/?src=api
Once you have installed the add-on above go to Add-ons section found below the icon. Then click extensions and choose Language Pack install helper’s preferences. Next open up a language pack download site and choose your locale (I chose, for example, fi.xpi).
Now you should change your Firefox interface language. As seen from the above the xpi files have some letters like: fi.xpi. The fi part is usually a sufficient guess when it comes to figuring out the actual key-code of the prefered language. In any case that you are in doubt about the key-codes look for example here (see the line: Code2): http://www.science.co.il/Language/Codes.asp
Once you are clear about your key-code go to Firefox address bar and type about:config
You get treated with “This might void your warranty!” message but proceed regardless. Since we are only going to change the Firefox language we want to search for a string called: general.useragent.locale
Now go and change the string’s value by right-clicking it and choosing Modify. Type your language key-code to the box that opens. Exit and restart Firefox and your language should be changed.
NOTE. If you accidentally change the general.useragent.locale to something that does not exist then the language will default back to factory settings (if you started with an English browser then the factory settings will take you back to English).