I've tried deleting the extensions files and it still wont install, it just says "the add-on from addons.mozilla.org could not be installed because it does not match the add-on Firefox expected."
Maybe some application on your computer (antivirus, firewall?) alters the extension download and breaks it? You can try installing from http://adblockplus.org/ which doesn't have that check - but if the download is really broken this won't make you happy.