This is not true. Servo and Spidermonkey compile fine on windows using the mingw toolchain. For the MSVC toolchain we don't have that fully working, but obvious

This is not true. Servo and Spidermonkey compile fine on windows using the mingw toolchain. For the MSVC toolchain we don't have that fully working, but obviously Spidermonkey is not the problem there since the official Firefox builds use MSVC. Mostly we have to fix the build glue. The Windows port didn't ship yesterday because of technical glitches post-install. Servo works fine, but after installation it had some strange behavior we couldn't track down in time. It should come in a few days.