OS X and Windows have accelerated H.264 decoders that are available to programmers. Firefox is an application on each of these platforms. Firefox can use the native decoders like every other developer. They are already using a third-party's H.264 license (Adobe with Flash) to play this sort of video on many sites. I have a hard time seeing why using Flash is any different than using the built in.

For platforms that haven't paid the license, leave a plug-in way to do it and let others fill it just like they are letting Adobe do now.