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.

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What happens to the Firefox window when this happens? Does it go behind a different window, or is the regular home page content replaced with the new content (in that case, does the Back button work)?

Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

• https://support.mozilla.org/kb/how-to-use-full-screen

You can use the SearchReset extension to reset some preferences to the default values.

• https://addons.mozilla.org/firefox/addon/searchreset/

Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).

If you do not keep changes after a restart or otherwise have problems with preferences, see:

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

You can check for problems caused by a corrupted localstore.rdf file.

• http://kb.mozillazine.org/Corrupt_localstore.rdf