If you want to set the Firefox UI language (User Interface items like menus and settings area) to something other than English, here are two simple steps to get it done and enjoy Firefox in the language of your choice:

If your native language is not English then why not switch to the language which comes naturally to you for your normal tech activities. Below you can see a couple of screenshots with the UI language in Firefox set to Kannada.

Firefox Setting Screen in Kannada Firefox Setting UI in Kannada

Simple language switcher Switch language option