How do I start Firefox in another language than the default?

I want to temporarily use Firefox in another languages so that menus and everything else in another language. How can I do that?

I have tried

firefox -no-remote -UILocale en-GB

but that still loads Firefox with my default locale (I have en-GB locale for Firefox installed).

I also found https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/565209 but I dunno if it's due to that.

Goto menu item Adds-On, then Languages and disable the non wanted language.
You need to restart firefox, then.

Another solution: from command line:

LC_ALL=en_GB firefox -no-remote

in this way you change the environment of firefox, making it believe it is in en_GB locale, only for the given execution.

Obviously both the locale than the firefox support package for that locale should be installed, for this to work.

For example, to see firefox in french, the prerequisites are:

sudo locale-gen fr_FR fr_FR.UTF-8 && echo "Success"
sudo apt-get install firefox-locale-fr
LC_ALL=fr_FR firefox -no-remote

You need to install language as enzotib (+1) pointed it out first. Then you can switch between available locales.

To switching it easier, you can install addon like quick-locale-switcher.

You cannot switch to locale you don't have installed before. Firefox does not have all translated messages bundled in default installation. This will be unneeded waste of resources on many levels.