In particular, the current version of Firefox kept struggling to link within a 4GB address space. Hence the comments about "having had to back out and ultimately delay some important new features like SPDY": nothing about SPDY itself, but it happened to push the link above 4GB and cause builds to fail, so it got temporarily dropped.

Thanks for the link. It appears the actual build system will still be based on make but just requiring the toolchain inside vs2010 (currently you can use vs2005-2010). Am I correct here?

Which means Server 2003 RTM is also dropped, BTW.