Okay, here's the solution. This should really be on the Firefox and RealPlayer sites.
All internet addresses which begin with rtsp:// are to be opened via RealPlayer. Let's tell our browser to do so.
We have to create (only if doesn't exist yet) a file called user.js in the user's Firefox directory and add some lines. Mine is located at ~/.mozilla/firefox/1yxw9wmb.default/user.js. It should be put in the same directory where prefs.js is.
And then say yes to the dialog box that pops up the first time you call up using RTSP.
This, simpler solution may also work for some.