I love Multirow-Bookmarks Rated 5 out of 5 stars
Multirow-Bookmarks is one of my favourite add-ons. It keeps what it promises, it's easy to handle, it simply works. What a shame it's not (yet) available for Firefox 4.
The suggested Roomy Bookmarks Toolbar (https://addons.mozilla.org/en-us/firefox/addon/roomy-bookmarks-toolbar/) does by far not replace it.
Beta Version of Multirow Bookmarks Toolbar is now available!
Come check it out!