I have this problem with only one profile on Win 7, while a clone of this same profile on WinXP never has the problem. All profiles have AdBlock Plus installed, so I don’t think that it’s that. On the Win 7 machine I have 4 profiles across 3 users, and on the WinXP machine there are 8 profiles across 6 users.
The Bookmarks Toolbar is shown but sometimes doesn’t populate with the bookmarks. Usually, restarting Firefox corrects the issue.