The best way is to download the language you want from '''[http://www.mozilla.org/en-US/firefox/all/ Download Firefox For All languages And Systems]''' {web link}

Thank you greatly. This is so frustrating that it's simply impossible to change language in settings. This [https://support.mozilla.org/en-US/kb/use-firefox-another-language article] is misleading — there were no drop-down ever since I've first switched to Firefox 3 years ago, and nothing changed in years even though I'm quite sure it's very common problem. I've tried so many different ways and the only working option is to simply reinstall the browser.