É uma extensão muito boa para traduções que não precisa de nuvem, mas é uma pena que não é possível instalar no Firefox Beta e Nigthly do Android.
Mas fico no aguardo de que tenha suporte para estas versões.

Very good that a local translation without cloud is possible because of privacy, but doesn't work completely. "Translate selection" in context menu doesn't work, nothing happens after selecting text and selecting this option. Also after downloading TranslateLocally to enable faster translations, there is an app with a translation window which works, but the extension doesn't seem to be impacted, nothing seems to change there (maybe it is actually faster, not sure) and you still can't choose TranslateLocally as a translation provider in the extension settings, it still says: "You can download TranslateLocally for faster private translations." Could it be that these things don't work because I use a windows 7 PC? I had downloaded the older compatibility version. If these issues are remedied I would give 5 stars.