I was looking for a simple "highlight, right click, translate" addon for Firefox and I found this one. Only problem was that it only translated from English to Spanish. I modified the addon so that it auto detects the language and translates to English. The voice translation only works for English text, but if you change the "en" in the url to whatever language the text is, it should work. Download it, don't, I don't care, just thought I'd share it.

You can download it here. Extract the xpi and drop it into Firefox to install. (.xpi is not an approved extension 'round these parts.) After that, highlight some text, right click, and translate. All credit to the original author, blah, blah, blah. I just changed a few characters...