There have been a few people in recent weeks complaining that their settings don't 'stick.' All have been Windows users, though that could simply be because that's what most people use. Have you read this: https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save When I last used WIndows (XP and Vista) permissions weren't much of a thing but I wonder if your profile directory has somehow become read-only so the settings can't be saved. I'd have expected either Firefox or Windows to show an error message in that case but what you EXPECT and what programs DO aren't the same thing. You could try right-clicking your profile directory, choosing Properties and whether it's read-only or not will be listed under Attributes and you can un-tick it if so. If you've got a backup it may be worth completely deleting your profile directory and making a new one, assuming the problem isn't in the back-up copy of course. I know that if I copied something off a CD-ROM the copy on my hard disk would always be read-only.