Never used VLC, but mplayer doesn't play everything, even with the latest additional codecs (e.g. from the mplayerhq.hu site).
Apologies if teaching you to suck eggs
, but don't forget to configure the mplayer-plugin once installed. Either edit your '/home/<username>/.mplayer/mplayerplug-in.conf' file, or use the GUI by opening a web browser window that will start the mplayerplug-in and right-clicking on the mplayerplug-in window, then selecting Configure from the pop-up menu. There you'll see a bunch of media support options, not all of which are enabled by default. Plus you can remove conflicts with other browser plugins you have installed (e.g. realplayer) that address the same media types.
If you have sound problems, change the sound system in the Audio Output box, and similarly the Video Output box if video problems occur.
You can experiment with the RTSP /HTTP selections as well and let me know if that does anything useful for you (it hasn't done much for me so far) - looks like that's more for firewall compatibility than anything else.
You can write a small shell script that copies plugins from the /usr/lib/mozilla/plugins folder to the /usr/lib/firefox-2.0.0.X/plugins folder every time firefox gets updated (and deletes the plugins).
I'm sure there's a more elegant way to do that (i.e. survive yum firefox updates and preserve all your plugins), so if you know of one, please post the solution.