Does what it says and does it well. Rated 5 out of 5 stars

Does what it says and does it well. For those wanting UK English but it keeps defaulting back to US English, perhaps you should have chosen UK English when installing Firefox but anyway all you need to do is go into Tools/Options/Content Tab/Languages/Choose/Language in order of preference: English/United Kingdom [en-gb] remove the US English if you wish, otherwise make sure UK English is the default then click OK.

This review is for a previous version of the add-on (1.19.1).