Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

One user who had this problem was using Firefox through Sandboxie. Changes made while running in Sandboxie apparently are not permanent.

There are a variety of reasons that changes to Firefox might be lost. This support article lists some of them: How to fix preferences that won't save. Some security/utility software also may "protect" you from unwanted changed by blocking all changes, even ones you make. If you use Advanced SystemCare's Surfing Protection feature, or similar features of other software, you may need to adjust or disable them.

Thanks for the help. The scan with Ant-Malware had some findings. Solving this fixed the problem.

Run several or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.