I'm still running Firefox 12, so if this is a new feature of Firefox 13, I am not familiar with it. It also doesn't sound like a standard Windows XP feature. It is possible that you accidentally installed some adware that tries to sell you a third party product. You may want to run some supplemental security scans. These two tools are highly regarded: Malwarebytes Anti-malware : http://www.malwarebytes.org/products/malwarebytes_free SUPERAntiSpyware : http://www.superantispyware.com/

See https://support.mozilla.org/kb/Firefox+consumes+a+lot+of+CPU+resources

Sounds like malware, check out the tools listed here: https://support.mozilla.org/fi/questions/927290