You're right on the former, I was under the impression that it was an economy-of-effort thing rather than an ideological thing. However, I'm not sure I agree that it would be as hard as you think. The mime types are registered here[1] and the impls are here[2]. Each media type is implemented as a state machine that could wrap ffmpeg. That's where I would start, I suspect.
EDIT: Looks like someone's already blazing that trail[3]. They're using gstreamer, but the research they do for that will show anyone how to add a new one.
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1) http://hg.mozilla.org/mozilla-central/file/b27f05c8743b/cont...
2) http://hg.mozilla.org/mozilla-central/file/b27f05c8743b/cont...