A new addition to Fx 3 which will add about 100-200 KB to Fx Code will be support (though not yet documented) for the forthcoming <VIDEO> <AUDIO> HTML 5 Tags. However, this currently not in any released (or nightly) Fx 3 builds and further:

Right now and for its first iteration, support will be limited to Theora and Vorbis, a couple of open source video and audio encoding formats respectively, developed by the Xiph.org Foundation, as recommended by the specification but other formats may be supported in the future as well.

News Source: Mozilla Links