Many users have posted about problems with the McAfee Site Advisor extension in Firefox 4.

McAfee site advisor has a post on this. Here is the link- http://service.mcafee.com/faqdocument.aspx?id=TS101144

@brinda : the link you provide is broken. Do you know the title of this FAQ document? I think it's been renumbered.

Edit : It's okay. That document referred to a particular problem with Firefox 4 which appears to have been fixed. The document has probably been withdrawn.