There may be a problem with one of the files that stores details of installed extensions. Try deleting these files as shown in the "Corrupt extension files" section of this link - https://support.mozilla.com/kb/Unable+to+install+add-ons

Though that article is not specifically about your problem, the method shown in the corrupt extension files section may help. When you restart Firefox after deleting those files, the list of installed extensions is rebuilt and this will hopefully fix your problem.

I had the same problem after installing Firefox 4. I fixed it by following the "corrupt extension files" suggestion.