Search bugzilla & you should find open tickets. They shipped first for platforms with native support, understandably, but last time I checked the intention was to support everyone.

EDIT: It looks like Linux support shipped in FF26 as long as GStreamer supports it: https://bugzilla.mozilla.org/show_bug.cgi?id=794282#c101

Windows XP is still left out – they use DirectShow for MP3 (https://bugzilla.mozilla.org/show_bug.cgi?id=861693) but using it for H.264 would require a licensed plugin which few people have and while OpenH.264 is available, it doesn't support the Main & High profiles used for most video on the web (it's been focused on streaming): https://bugzilla.mozilla.org/show_bug.cgi?id=799318#c57

It's irritating that they're not just pushing all media through DirectShow, since that's the Windows native framework for doing it and it automatically supports whatever codecs are installed. Their excuse is that they don't have enough control over what codecs users install, which is a bit disappointing.

ffdshow has been able to play H.264 and a bunch of other codecs for a long time, and a not-insignificant number of users probably have that or some other codec that can...

Media codecs are a very common source of security holes, which is one of the main reasons for not indiscriminately using just any codec that might be installed on the system. I think most users value the decision to not unnecessarily expose them to these security holes.

Also, that wouldn't change anything about the current situation: we still can't rely on users having the right codecs installed. We're not saying that Linux and Windows XP won't be supported going forward. They most definitely will. As part of this first small step towards deployment, we opted for a conservative approach where we only enable Shumway in situations we have the maximum amount of control over. This will be broadened over time.