Puntuado con 4 de 5 estrellas
I like it but I wish you could clear the popup by simply clicking outside of it (instead of finding the word 'Close' in small text).
Also (ideally) I would get rid of all the text outside the dotted lines, and give a little more white space around the definitions. I think it would make it much easier on the eyes this way.
Btw does anyone know if this guy is still working on it? I was surprised to see that it was functional in FF3, but last updated in 2006 (there's no developer's page from the add-on).
To the previous poster--you could also add a thesaurus (Thesaurus.com?) to your search engine bar with Add to Searchbar, if you haven't already. [And use ContextSearch to add it to the popup menu.]).