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If you suspect malware please scan with all the tools listed in the article

• Troubleshoot Firefox issues caused by malware

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
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• 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. Windows 8 has antivirus built-in already.

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

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You may for instance have the Potentially Unwanted Program mentioned here

• http://www.lavasoft.com/mylavasoft/rogues/help

I would not recommend you check the Firefox addons, and the programs listed in Windows Control panels and uninstall anything suspicious and unwanted. I would not suggest you try hacking any Registry entries directly without trusted and detailed advice.

Scanning with the tools mentioned is still a good idea. Once one piece of adware or malware has a toe hold others often arrive and proliferate.