Or maybe, just maybe, Mozilla prefers a royalty-free, patent-unencumbered standard instead. H264 is neither. Vorbis/Theora is both.
Imagine for a moment what the web would be like if you had to pay royalties for using HTML. This is what Mozilla and Opera are trying to prevent. Not only do they have to pay for including a decoder, site owners would also have to pay for distributing H264-encoded video (and they pricing structure is far from straightforward.)
Don't think this could ever happen? It's already happened twice, first with GIF, then with MP3. Those who forget history are doomed to repeat it.
All I can say is kudos to Mozilla for standing up for the web.