Microsoft's Atlas framework for AJaX got some harsh comments from Microsoft's partner Wintellect about the lack of cross-browser interoperability. At the end of the day, AJaX really came up because tht different component frameworks and client capabilities are so disjoints, that for a long time there was no way you could build a rich Web UI. With Atlas only supporting IE (for the interesteing parts, at the very least), the benefits of AJaX go away.

So if Microsoft is truley serious about making Atlas a usable AJaX framework, they will have to support Firefox and Safari, at the very least.