H.264 isn't going away anytime soon since unlike WebM it can be decoded via
GPU.
Mozilla wishes to expand its browser's market share on mobile devices which
rely on video decoding via GPU, so they must support H.264 via HTML5 on their
mobile versions, since they cannot support it via the Adobe Flash plugin as
Adobe dropped Flash on mobile devices (primarily due to Apple refusing to
ship Flash on their mobile devices).
Mozilla cannot reply on Adobe to provide its plugin to desktop users either
as they are dropping support for GNU/Linux users, so by providing H.264 via
HTML5 Mozilla's desktop users will be able to playback high definition
content via GPU if their GPU supports it.
