OK, didn't register my first review... so again: I love TheFreeDictionary.com. While some sites (dictionary.com) give you a popup ad every new look up and have limited information, this site gives you so much more. Thank you Farlax for making the site and 'fertavares' who added this search add-on. Also, to the reivew by Les, it may not be compatible with firefox 4 if that's what you have, not sure since i'm still on the old version...
This is just what I was looking for: the add-on for search engines bar in FF, unlike the right click tool I installed before. Installed without any problems, as all other add-ons I tried, and works fine.
The only improvement I could think of would be opening search results in a new tab, instead of the same window. Otherwise perfect.
I love this dictionary. I find it more useful than webster's and dictionary.com because it lists more medical and science terminology. It also lists the 25 words that come alphabetically before and after your target word, for those times when you can think of part of the word but not the exact phrasing.