Just thought I would revive this old thread because the previous information is now incorrect and obsolete.
Up until now I always kept my version of firefox at version 5 or below, that way all my installed add-ons kept working.
However, I recently allowed firefox to update to the current version of 23.0.1 to see what happened and if there was any major difference only to find all of my add-ons still work, although a couple of them do have to be updated, other than that I haven't noticed any difference.