How does handing it off help? The problem with h.264 is that it's proprietary and patent encumbered; that's not fixed by handing it off to anything.
It helps because Firefox doesn't have to implement h264. It can simply ask a system service to play the video, something which the vast majority people are legally entitled since they have already paid for h264 as part of their OS licence or software patents don't apply where they live.
Secondly, if Firefox is so uptight about proprietary / patent encumbered things, I suggest it outlaw all plugins outright rather than play some silly double standards game. Ban any plugin that plays Flash, PDF, Java or anything else and see how far this gets them.
Thirdly, supporting Theora and only Theora is self defeating for Firefox. Sites and users will simply ignore the browser, or hack around the limitation by using Flash. Either way Firefox loses.
All of which can be avoided simply by opening up the APIs. If they don't, well... so long Firefox.