Firefox on Linux has GStreamer but it's turned off at build time. Developers can build with it to test it. At some point it'll be enabled by default, or distros can enable it for their builds, to get H.264 support.
On Linux Firefox will use GStreamer, the same backend also used in Firefox OS, its not enabled in the nightly builds since not all distros have the right version of GStreamer and it ca not be loaded dynamically right now (https://bugzilla.mozilla.org/show_bug.cgi?id=794282)
will you enable H.264 support only on desktops once you have working backends for Windows/Linux/OS X or will they be enabled by default as they are working? Just wondering if there will be stable releases of Firefox where some platforms have H.264 decodes and some don't.
They will be enabled by default as they are working. We already have the issue of some platforms having it and others not. Some Android devices have it with Firefox for Android and some don't for example. Linux distro's could enable the GStreamer backend and gain it while others don't as another.
For detection sites should use the API "canPlayType" and similar functionality to detect if the browser supports playback.
It was a pretty big deal a while ago that quicktime support H.264; off hand, I'm pretty sure they supported it for years before MS did. If firefox supports quicktime, they support H.264.
EDIT: Well, I'll eat my hat. I'm not sure why Firefox needs a different licensing agreement on OSX when they COULD just hook into quicktime without any licensing fees at all. I'm probably grossly misunderstanding the situation.
EDIT2: Ok, so I guess AVFoundation is just the objective-c frontend for OSX's media libs. I can't for the life of me understand why it took them so long to implement.