The vast, overwhelming majority of video watched since then has been H.264 via Flash (since it was more "pragmatic" to do so), so I struggle to see how that would have affected Mozilla in any major way. If flash had adopted VP8, (like they said they would) then things might have been very different.

And if your talking about mobile browsers, smartphones are dominated by two platforms, each owned by the corporate parents of two webkit-based browsers, one of whom doesn't allow you to install other browsers at all.