Perform the suggestions mentioned in the following articles:

• Clear Cookies & Cache
• Clear the Network Cache
• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Check and tell if its working.

-> Update Firefox to the latest version (9.0.1)

• Update Firefox to the latest release

-> Update All your Firefox Plugins

• https://www.mozilla.org/en-US/plugincheck/
• When Downloading Plugins Update setup files, Remove Checkmark from Downloading other Optional Softwares with your Plugins (e.g. Toolbars, McAfee, Google Chrome, etc.)
• Restart Firefox

Also do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked