Probably only for visibility. Or MS is in the process of sponsoring them.
They should restructure instead, hire people who actually want to work on software and not use corporation and foundation around it as platform for their... peculiar "endeavours". But I doubt that's gonna happen - flow of Google cash and from all those naive people who think supporting Mozilla directly contributes to Firefox is too good it seems. But then it's understandable they do this - money from Google tap can get twisted.
Think you might be on something, with the incoming end of Google cash flow, Firefox may be in discussion with bing and that could be part of the agreement, use Microsoft server.