Not needed in FF3... Rated 5 out of 5 stars
...as Dominic Pfluger stated in his comment, it's obsolete. This feature has been incorporated in the FF3 'Bookmarks Organizer'. Open the 'bookmark Organizer' feature from the bookmark menu. Highlight a bookmark, then at the bottom of the editing area click on the 'More' button. This will open the 'additional text editing box's/dialogs' which work and look just as they did in the 'Flat Bookmark Editing' extension. This was and still is an excellent extension for users of FF prior to FF3.
Regards and thank you "chouser" for a great extension. Emulation is the greatest honor, and FF3 has done just that for "Flat Bookmark Editing and it's developer"! ;)