Rated 3 out of 5 stars
Add-ons don't honor locale switch
More specifically, they will switch to a locale you specify, but they will not switch back.
Switch to es, restart, all menus are in Spanish. Switch to en-US, restart, Firefox menu items are in English, Web Developer, Firebug, Quick Locale Switcher(!) menu items are still in Spanish. The only way to fix this is to disable Quick Locale Switcher. FF3.5.3 Windows XP.