The only one I use at the moment is:
Define WordAfter trying a few similar tools, I decided to keep this one. It's simple, flexible, doesn't take any of my valuable screen space, and I can choose/add as many dictionaries or translation services as I want. I prefer this because I can choose the resources I consider "best" and have them available at all times.
How about you? What translation-related firefox add-ons do you use and why?
