Comment 8 for bug 225425

The exact same problem that Francesco reports above has happened to me a number of times; and often I'll then need to reset. After a bit of googling, I found this page. I'm using Firefox 3.0.1 under Xubuntu 8.04, and have attached my about:config, as John suggest.
A workaround is of course to use the "Organize Bookmarks" instead, which has never caused any problems.