If it stops working try the Dev Build Rated 5 out of 5 stars
I just upgraded to FF 13 (beta) and 'Insert Bookmarks to the Top of Menu' stopped working and would save the bookmark somewhere near the bottom of my bookmarks folder which was annoying. I installed the Dev build and it works correctly again now.
You can do this by clicking on Support Site on the right side. It will open up the mozillazine forum page. In the first post look for Install/Download and then click on Latest DevBuild. It will open up a mozilla forum page in chinese. At the bottom you will see the link for Add Bookmark Here ² for FX13.0+.