Rather than uninstalling/reinstalling, I'd check out if the profile is corrupt or some plugin is messing it up. I don't think reinstalling would fix that. Certainly not downgrading.

You could test plugins by removing/disabling them. If that doesn't work, then you can test the profile by making a temporary user account in XP, log in there, and see if firefox still does that with the new account and clean profile. If it still does, then something more fundamental might be broken or you have a weird virus or malware installed. If not, then you'll be looking at deleting the profile in your user account and starting afresh. You might want to export your bookmarks in this case, ready to import with the new profile so you don't lose them, and maybe save plugins. The profile to delete should normally be in C:\Documents and Settings\{username}\Application Data\Mozilla\Firefox.