Rated 5 out of 5 stars

Great add-on, that sadly does not work with FF4, an alternative to this add-on is the aforementioned userstyle or the roomy bookmarks toolbar add-on.

This review is for a previous version of the add-on (5.0). 

Beta Version of Multirow Bookmarks Toolbar is now available!

Come check it out!
https://addons.mozilla.org/en-US/firefox/addon/multirow-bookmarks-toolbar/