FC5 has adopted a project called gstreamer as a common back end for handling video and audio files. Totem, the standalone multimedia player shipped with FC5, uses gstreamer; so does your Firefox plugin, mozplugger.
gstreamer itself uses plugins, one for each multimedia format it supports. Plugins for open-source formats such as Ogg Vorbis ship with FC5. Plugins for proprietary formats don't. To get these, do this:
yum install gstreamer-plugins-ugly ffmpeg
This may also install mpeg2dec, libsidplay, libdvdread, faac, faad2, imlib, and gsm.
The 'ugly' plugins enable gstreamer to handle MP3, MPEG2, and SID formats. The SID format is a historical relic from the Commodore 64. MP3 is the most popular audio format. MPEG2 is an audio/video format used in DVDs.
The ffmpeg library can encode or decode MPEG1 audio and video, MPEG4, h263, ac3, asf, avi, real, mjpeg, and flash.