(Old description follows.)
Allows you to change the default menu items of Firefox, including the right-click menus. Mostly useful to remove things you never use. Based on the original "Menu Edit" v. 184.108.40.206, the developers of which seem to have gone AWOL.
It currently doesn't handle the contents of the bookmarks and history menus. (You can hide those menus entirely, though.)
I'll probably only add functions for that after the FF3 release, in case they change.