Odd Firefox system requirements recommendation

Maybe I'm just focusing on nothing, but Mozilla's system requirements recommendations for Firefox 3.0 beta struck me as odd:

Anyone any ideas why XP would be recommended over Vista?  The same pattern is repeated for Firefox 2 and Thunderbird 2 too:

I guess it's easier to understand why this applies to Firefox 2, but for the same recommendation to be in place for Firefox 3 seems odd to me.  I've got the beta running just fine on Vista.

I wonder whether the recommendation isn't more political than technical.

Anyone able to shed any light on the matter?