Firefox does not support MPEG-4 video because of patent licensing issues. Firefox supports Ogg files with Theora video and Vorbis audio. Google has an agreement

Firefox does not support MPEG-4 video because of patent licensing issues. Firefox supports Ogg files with Theora video and Vorbis audio. Google has an agreement with MPEG LA and is therefore able to ship an MPEG-4 AVC decoder with Google Chrome.