Well, this problem has been occuring for about two days. I thought I stopped it, but I was wrong.
Firefox is constantly erasing my private data (including my history). I've unchecked the box that erases private data when Firefox closes, but whenever I open Firefox, it's checked again. Reinstallation doesn't help either. Is there anything I can do? Thanks in advance!

Backup your bookmarks. Don't backup anything else (such as extensions) as they may be causing the problem.
Uninstall Firefox again.
Then delete the Mozilla Firefox folder in the Program Files sub-directory
Then follow these directions to delete the rest of the stuff that Firefox leaves on your hard drive: http://kb.mozillazine.org/Uninstalling_Firefox
Reboot and reinstall a fresh copy of Firefox.

If you're installing extensions, install them one at a time and test each one before installing the next one (in case of a bug in an extension).

Hmm, thanks for the advice, but the problem's faded out: this is quite strange, I don't see why this happened...