OS X include a H.264 decoder? Check.
Windows include a H.264 decoder? Check.
Mozilla release products for OS X and Windows? Check.
Not really sure what the issue would be.
I'm not sure I have this right, but I'll take a stab at it.
OSX includes h264 decoding as part of quicktime, in order to view an h264 encoded video in the firefox browser, one would have to embed a quicktime movie - therefore using the quicktime plugin for the browser, like so:
Code:
<object classid="clsid:02BF25D5-8C17-4B23-BC80-D3488ABDDC6B" codebase="http://www.apple.com/qtactivex/qtplugin.cab" height="256" width="320">
<param name="src" value="http://www.yourdomain/your.mov">
<param name="autoplay" value="true">
<param name="type" value="video/quicktime" height="256" width="320">
<embed src="http://www.yourdomain/your.mov" height="256" width="320" autoplay="true" type="video/quicktime" pluginspage="http://www.apple.com/quicktime/download/">
</object>
That's not the video tag.