OK that isn't the latest version. Click to the right of the icon again and select "Get latest version". If it updates OK close and reopen Firefox and see if the problem persists.
If it doesn't update or if the problem persists, go to Control Panel/Add or Remove Programs (Programs/Uninstall a program in Vista) and uninstall it. If you don't see McAfee SiteAdvisor listed, look for McAfee Security Center, select it and then select only SiteAdvisor for uninstallation.
Reboot and then reinstall, either from your account or from http://www.siteadvisor.com
Hope that helps.