If you close Firefox, then start Firefox in Safe Mode, does the problem still occur? For instructions on starting in Safe Mode, see Diagnose Firefox issues using Troubleshoot Mode.

Yes it still occurs in safe mode and normal setup. I upgraded to 3.6.4 and it still happens.

Try to rename prefs.js to prefs.js.sav in the Firefox Profile Folder If renaming the file works then you can delete the corrupted file prefs.js.sav. --- Delete localstore.rdf or rename to localstore.rdf.sav in the Profile Folder to test if the file is corrupted. See http://kb.mozillazine.org/Corrupt_localstore.rdf (caution: do not delete the localstore.rdf file in the Firefox program installation folder)

Note: Deleting the file localstore.rdf will reset the customizations of the toolbars to the defaults. You can rename localstore.rdf to localstore.rdf.sav to test if that solves it. Then you can restore the customization by copying localstore.rdf.sav to localstore.rdf if it didn't work.

I tried the suggestions by cor-el. The problem with my bookmark has not changed.

Your Firefox settings and info are kept in a folder separate from the Firefox program, called your profile folder. Try creating a test profile, to see if the problem still occurs. See Profile Manager - Create, remove or switch Firefox profiles

I made a profile called Test. The bookmarks worked fine so I imported my bookmarks. When I did this I had the same problem in the new profile. Are my bookmarks corrupted? Can I salvage them?

It depends how you imported them. Did you use the import feature, or did you just copy the file over?

Firefox makes an automatic JSON backup of your bookmarks on a daily basis, so in your original profile, you could restore the backup using the instructions in Bookmarks and toolbar buttons not working after upgrading

I tried your suggestion, but no html file is exported. I tried a couple of times with the same result.

Do you see any bookmarks in the sidebar in the old profile? If you see them then try to copy them to the clipboard and paste them in the other Firefox instance in the Bookmarks Manager or sidebar.

You need to start the second Firefox instance (profile) with the -no-remote command line switch. See http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile