In fact, there's this bug for H.264 support: https://bugzilla.mozilla.org/show_bug.cgi?id=541286
It was raised in January, and it's status is: "RESOLVED WONTFIX"
The stable Firefox build, as of today(12/19/2012) does not support those codecs.
- support for Linux via GStreamer landed but not built by default. It needs a bit more work.
- support for Windows 7 and above which the OP post is about
- Mac OS X is not yet underway but is planned.
- Hardware H.264 decoding on Android phones via libstagefright. Some phones supported and enabled by default. Some supported but disabled at the moment. Some unsupported.
- Hardware H.264 decoding on Firefox OS/B2G enabled by default, implemented via libstagefright. Phone vendors provide the binary blobs to do the decoding.
For detection sites should use the API "canPlayType" and similar functionality to detect if the browser supports playback.