How to Change Firefox Language

If you want to change your Mozilla Firefox locale, ie from Spanish to English, then write the following string in your URL bar: about:config

Once there, search for general.useragent.locale and change the current value from es-ES (or whatever) to en-US. Then, restart your browser.

But if you want to translate all menus, labels, etc. then probably you may install the Language Packs from Mozilla.