Great fix by Bas! Rated 5 out of 5 stars

Re: BookMarkToolbar fix for FF4.0b8
Inside your Firefox profile, navigate to extensions\{7bf7c1c3-d8e7-4020-ab38-a829c76693d5}\chrome\placesfulltitles\skin and open placesfulltitles.css in a text editor. Do a find-and-replace: replace all instances of
bookmarksBarContent
with
PlacesToolbar
and then close the file and restart Firefox.

This review is for a previous version of the add-on (3.2.4).