Kate it could be that your profile has become corrupted in some way. The usual way to try to eliminate this is to create a new profile and then add the add-ons back one at a time, it at some point in doing this it breaks then that is usually the add-on at fault. It may of course not break and all will be OK. The problem of a new profile is that you will need all your passwords and bookmarks, these are still there in the old profile. You start the profile manager using the command firefox -P from a terminal (or DOS) window, this allows you to create a new profile and to select the one to make as default which starts when you start firefox normally. To save you bookmarks you do this from your existing profile and use "Show all Bookmarks" from the menu, passwords are more tricky but there are add-ons which allow you to export them.
Stuart