It's a great addon, however, the auto-detect feature has a SEVERE flaw. If there's more than one language, it fails to translate at all.
The following example is german with several english words.
"OpenSAGA ist eine Open-Source-Software-Plattform zur effizienten Entwicklung moderner Webanwendungen." Things like that will fail to autotranslate, requiring you to manually set languages. It's not a big deal, it's just a real PITA.
This addon also seems to have problems with frames in webpages. Anytime it tries translating inside frames, firefox will hang until you get an option to stop the script or let it hang again.
Even with those flaws, this is an essential addon for me. None of the others I've tried work half as well as this one, and I'd definitely recommend it - with some explanations about its hangups.