As a native Spanish speaker who works for an English website, I use online dictionaries and translators quite often. And my favorite above all is WordReference. This is why I was excited to see a Firefox add-on especially designed to make WordReference even easier to use!
WordReference Translator is a small plug-in that lets you translate any word you highlight with the mouse on the website you're currently visiting. All you have to do is double click the word and WordReference Translator will overlay a small white pop-up window with a link to quickly translate the word.
The good thing about is that it supports all the language combinations of the original website, which gives you plenty of options when translating. Also, the dictionary is supported by a very active user community and forums where you can solve almost any doubt you have about certain translations.
On the downside, to be honest, the pop-up link is not very usable – it's semi-transparent and hard to see – but you can also translate the selected word via its context menu. In both cases the translation opens up in a whole new tab, instead of displaying it on that same pop-up window.
Despite these minor usability issues, WordReference Translator is still a handy companion for those of us who are frequent dictionary users.
If you love WordReference as much as I do, you'll find this Firefox add-on really helpful.