How to change default external protocol in Firefox

Generally if you click on a rtsp:// link from Firefox then a popup appear displaying "External Protocol Request" with default player as totem to play that streaming media. But if you want to run a media with RealPlayer or with other software then do the following:

1. Open firefox and type following in the location bar:
   about:config
   
2. Right click any where and select New->String and type following name:
   network.protocol-handler.app.rtsp
   
3. Enter following value if you want to play it with RealPlayer:
   /usr/bin/X11/realplay

Now whenever you will click a rtsp:// media, RealPlayer will appear instead of default Totem player. To check with a media go to http://m.vtap.com and click a video to see it in an action.