Regardless of which browser is your default can you advise what version of Internet Explorer is installed please? Also is it up to date with all Microsoft updates?
The reason I ask is because McAfee's UI is in fact an I.E. page, albeit an internal one.
If you have IE8 installed (which you should have unless you are using Windows 2000) go to Tools/Internet Options/Advanced and click Reset and OK any prompts.
Close and reopen IE. Go to Tools/Manage Add-ons and re-enable the add-ons.
If it continues contact Technical Support Chat via Useful Links above.
Have you by any chance been using registry cleaners lately? They also can cause this sort of thing.