See:

• http://kb.mozillazine.org/Preferences_not_saved

The same problem.

1. Set Network settings to No Proxy
2. Disable addon Screengrab
3. Restart Firefox

I have Screengrab 0.96.3, XP SP3, FF4. Disabling helped me.

Under your

Tools -> Options -> Advanced -> Network (tab) -> Offline Storage -> Click the "Clear Now" button.

Then click the "Settings" button and check the box for "Auto detect" and click OK

Lastly remove Screengrab by going to Tools -> Addons -> Extensions -> ScreenGrab -> Remove

Restart FireFox and reinstall ScreenGrab. =)

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I had a virus, it could be triggering this: Backdoor:Win32/Cycbot.B is a backdoor trojan that allows attackers unauthorized access and control of an affected computer. After a computer is infected, the trojan connects to a specific remote server to receive commands from attackers. The commands may include instructing the trojan to update itself, visit web links, or download and execute arbitrary files. I would advise to check your system

I had this too, I tried running Firefox in safe mode (no add ons) - that didn't make any difference. I could save other preferences, just not the proxy setting. I'm 99.99% sure I didn't have a virus or other malware. This was only affecting Firefox, no other browser I has installed.

In the end I created a new profile - http://support.mozilla.com/en-US/kb/Managing%20profiles - that seems to have 'fixed' it for me - the new profile doesn't have the problem, but the old one still does.

Windows 7, Firefox 4.01.

Cheers

Same happens with Firefox 7.0

İf you people had the same problem that i had minutes ago with firefox after restarting firefox it gives error something like: ___________________________________________________

De proxyserver weigert verbindingen

 Firefox is ingesteld om een proxyserver te gebruiken die verbindingen weigert.

 Controleer of uw proxyinstellingen juist zijn.
 Neem contact op met uw netwerkbeheerder om te controleren of de proxyserver
   werkt.

___________________________________________________

simpely you uninstall firefox with advaced software like Revo Uninstaller completely with all register stuf thats been normaly left behind and install it again.

not to lose your bookmarks just use addon cold Xmark ;) .