There's An(other) App For That Rated 5 out of 5 stars
If this happens to become unsupported and obsolete, there's a very similar add-on called the Element Hiding Helper for AdBlock Plus. It also stores rules for permanent removal of page elements, and it also uses a WYSIWYG approach and has already been updated for Firefox 4.
Also, NEVER download and install an unverified XPI file from an unknown source (e.g. box.net)! You could be walking right into a malware trap. How do you know what's been done to that XPI file? If all you need is a version bump, there are automated tools, like MR Tech Toolkit, that can force compatibility with new FF versions. Of course things might break doing it, so caveat emptor/YMMV/etc.