It says: "Due to late changes in the Cross Site XMLHttpRequest specification which made our implementation incomplete, it was decided to remove support for this technology rather than include only partial support." Since this was posted today, this would apply to the upcoming Firefox 3 Beta 5, but not for the currently available Beta 4.
Does this mean that cross-site scripting will be completely non-functional in Firefox 3? Does this mean that McAfee SiteAdvisor won't work with Firefox 3?
As I understand it, that was to be something new in Firefox 3. As McAfee doesn't support anything in its beta stage anyway I doubt that you'll get an answer here. You could try using the online form to contact the team directly HERE. They do take some time to answer I will warn you.