Update.--June 22, 2011, Firefox has released version 5 but my test show the problem with some extentions and add-ons still persist.
I don't know what to tell you to fix this. I simply created a new user with basic verisons of the four extension files in the library for FF, found with Mac's at 'Users'/Library/Application Support/Firefox/Profiles/..., see the gobbly-gook folder name dot default. There you'll find the four extention.'something' filename. Just close FF, move those file, open FF - it recreates empty ones, and roll on.