Although Firefox is your browser of choice, McAfee software utilizes Internet Explorer for all of its functions, so open IE (which I hope is IE8, the most secure version available for XP, if not I suggest installing it) and go to Tools/Internet Options/Advanced and click 'Reset', Apply and OK. Close and reopen IE and go to Tools/Manage Add-ons and re-enable any you may needs, at the same time make sure that the usual ones, Adobe Flash and Shockwave and Java aqre all the latest versions.
If that doesn't help, then it's probably easiest and quickest for you to get Technical Support to check it out. It's a free phone call or you can use online chat.
Oh dear, then try the phone support. They are closed now but in the morning it should work: http://service.mcafee.com/ServiceSelect.aspx?lc=2057&sg=TS&pt=1&st=PHONE