See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

See http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

Didn't help. I restored bookmarks from backups; the add-ons, however, are still broken after trying the suggestions in your suggested "corrupt extension files" solutions. I switched to Firefox several years ago because IE was so problematic. Today it appears the tables have turned. IE still isn't perfect, but I don't have to worry about wholesale destruction of add-ons and bookmarks. Firefox, meanwhile, has been buggy the last several versions and this is the final straw. I think I'll just (sadly) forget that Firefox exists for the time being, as I know longer enjoy spending inordinate amounts of time on ridiculous problems that may have no solution.