In message <20050111201200.GA4811@gmx.net>, Sebastian Prause writes:
>mplayer should do that. also have a look at the -dumpstream option
>if you want to save the file
>

It turns out that there are two parts the answer.  The first I found 
with google's help: how to teach firefox about a new protocol type.
The answer is to add these lines to your user.js file:

user_pref("network.protocol-handler.external.rtsp", true);
user_pref("network.protocol-handler.app.rtsp", "/usr/pkg/bin/mplayer");

(Or realplay, if that's your preference.)  I'm not sure if the problem 
now is a codec -- here's the error mplayer is giving me.  (Realplayer 
just says "can't connect"; it may be a NAT issue, or it may need a 
newer version of Realplayer than we have in pkgsrc.)

   Connecting to server yyy.zzz[xxxx]:nnn ...
   rtsp_session: Not a Real server. Server type is 'SIPquest RTSP Server 1.0/2.0'.
   Not a Realmedia rtsp url. Trying standard rtsp protocol.
   RTSP support requires the "LIVE.COM Streaming Media" libraries!



		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb