[ZeroGravitas]

Realistically Firefox will play H.264 via Flash just the same as IE 6, 7, 8 and so will play any video on the net (including those in old Flash formats) till those 3 browsers collectively drop below 5% or so. By which time the conversation will be about H.26_5_ and a royalty-free successor to VP8.

[recoiledsnake]

The problem is that Firefox will not and won't load Flash for the HTML 5 <video> tag which references a H.264 video.

[ZeroGravitas]

The solution is that IE 6, 7, & 8 won't load Flash for the HTML5 video tag either, so the same Flash fallback (probably delivering the same H.264 file given to HTML5 browsers) works for all of these highly popular browsers which currently account for something like 80-90% of all browsers between them.

Firefox only has to worry about niche, tech-forward sites that feel they can disregard all pre HTML5 browsers and also actively choose to ignore Firefox (and Opera) as well by not providing a WebM fallback video. No ordinary business can afford to simply refuse to deliver a Flash video to 1/3rd of their audience when they've already built it and are serving it to another 1/3rd on older version of IE. (This obviously occurring at some future time when the HTML5 video delivery is preferred over Flash for any platform other than the Apple ones that don't have Flash, otherwise the Flash would be going out to 99% of browsers anyway).

There's not much downside for Firefox except pissing off people who really passionately hate Flash, but are quite happy with H.264's patent situation. I'm thinking the crossover is pretty small on those two populations and probably shrinking greatly now that Apple has let Adobe use their hardware acceleration API for H.264 decode, certainly not big enough to derail a browser used by tens of millions of ordinary people.