Symptoms include bookmarks appearing during first run, but missing after restarting Firefox. Bookmarks will show up if Firefox is reinstalled, but again disappear after restarting it. Solution is to configure Norton 360 according to our article http://support.mozilla.com/en-US/kb/Configuring+Norton+360
Lucy, do you want to take this one since you probably have a better understanding of where this should go? Thanks!
Added section about Norton 360. Need to double check if older versions of Norton AV also cause this problem. If anyone has any nits about my wording please go ahead and fix them when reviewing.