There r so many add-ons that can not match with fire fox earlier versions. These addons r for latest versions or firefox version 3.0 or later. This is simply firefox version problems. Download the latest one and then try for addons. Remember for safe game, take backups of ur bookmarks, if u have.
i agree with Rechil on this. there are some add ons that cannot be used on the latest version of firefox which is v 7.0.1.