Today I landed Firefox's WebM support on mozilla-central, our Firefox development branch. It should appear in nightly builds from tonight onwards.
Credits to the development effort go to Chris Double for writing the decoder backend, Matthew Gregan for writing the ISC style licensed libnestegg WebM demuxer, and myself for integrating all the moving parts into the Mozilla tree and build system. Also thanks need to go to the releng team which promptly installed YASM on the Mozilla Linux and Mac build machines, and to Roc, Shaver and many others for cracking the whip and working behind the scenes.
Firefox should build with WebM support without needing any extra changes to your build configuration, unless you're building on Win32, where you'll need to have MASM installed in order to compile libvpx's optimized assembly. MASM ships with the Windows 7 SDK, and with Visual Studio Pro. If you've got neither of those installed, you can also download MASM directly.
If you're building on Linux x86, Mac x86 or Mac x86_64 and you've got YASM installed, you'll automatically build VP8 decoder's optimized assembly code from libvpx. If you don't have YASM, you'll fallback to using the generic C code, which won't be as fast, but still performs acceptably. We don't have WebM support building on Win64 yet, you can disable if you reconfigure with --disable-webm.
If you're looking for some WebM videos to test with, you can view WebM videos on YouTube's HTML5 Experiement with nightly builds from tonight onwards.