This is not bad but has its quirks. I still have not found the perfect addon for my needs - a quick and lean in-line translator. This one works pretty well in terms of translation and the way the pop-up looks. I also really like that synonyms in the translated language are provided.

However, the implementation seems to be a bit intrusive. This addon sets cookies from Yandex.ru although I have deactivated the "ad-ware part" of it, which leaves an odd feeling about using this add-on. Also, perhaps because of my Firefox/Cookie configuration I am shown the introduction window every time I start Firefox. What's more, the translation of the UI has some errors (in German at least). Not a big problem but as this is a tool for translation, I wanted to mention it.