It only works in Windows 7 and above and the feature will probably land in Firefox 22
About a year ago, Mozilla officially gave up the fight and announced that it would be supporting proprietary audio and video formats in HTML5 <video> and <audio>.It had held out, supporting only the open source Theora and WebM, but it was the only one left in the fight and eventually decided that it was not worth preventing its users from making the most out of what's on the web.
However, it took a while to get from decision to implementation. In fact, support for H.264 and MP3 is only now enabled by default in Firefox.
"Support for playing H.264/AAC in MP4 and support for playing MP3 audio files in HTML5 <video> and <audio> elements has now been switched on by default for Windows 7 and later in the Firefox Nightly channel," Mozilla's Chris Pearce announced.
"You no longer need to set the pref to enable it. Please test MP4/MP3 support in Firefox Nightly builds, and file bugs in the 'Core:: Video/Audio' component in Bugzilla," he added.
That's only on Windows 7 and above and only in Firefox 21, which is currently in the Nightly channel.
Still, what it means is that in a few months, Windows users will be able to watch HTML5 videos encoded with H.264, which is most of them, or listen to MP3 streams without needing a plugin, though only if they have the proper codecs installed on their machine.
Firefox doesn't ship with support for proprietary codecs out of the box and never will, it will use whatever codes are available on the operating system it's running on.
The plan, for now, is to ship Firefox 22, scheduled for release on June 25, with support for H.264/AAC/MP3 support enabled by default, for Windows 7. Mozilla is working on implementing support on all the other platforms Firefox uses, desktop or mobile.