Mozilla does support it, but only if the underlying operating
system does. In OS X's case, they use VideoToolbox to implement
H.264, which was only partially implemented in 10.5 as a private
framework and not at all in 10.4.
Although I eventually plan to implement gstreamer or something
similar in TenFourFox, it will be a very big undertaking and it's a
long way off. In the meantime, the QuickTime Enabler can shunt some
of these video files to QuickTime Player, though not all.