One potential cause of this is an unwanted extension. Try disabling ALL nonessential or unrecognized extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable.

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

If you are adventurous about installing software from various websites, you may have loaded a bundle that contains some, well, crap. To review recent additions, go to your Windows Control Panel, Uninstall a Program, and click the "Installed on" column heading to bring the most recent items to the top. Remove anything unexpected or unwanted.

Success?

Start Firefox in Safe Mode and see if you still have the problem.

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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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