You installed Firefox 3.6 Beta, then uninstalled that and went back to Firefox 3.5.5?
The problem with Betas is that you never quite know what's going to happen. It may be that your Firefox profile is corrupted. If so, you might have to completely uninstall Firefox, delete all remnant Mozilla and Firefox folders, reboot the system, then reinstall Firefox.
John (Wilkinson)'s sticky thread here may help, at post #2.
I have no idea why but when i updated to Firefox 3.6 beta and then to 3.5 it never lets my save my open tabs when I close Firefox.
- and yes i did go into about:config and make sure that it is set to save the tabs when they close