Simpler fix (Score 2, Interesting)

Instead of installing the extension, you could instead do this:

1. Open the Javscript Console
2. Copy the following line-by-line into the textbox (hitting enter after each line):
   
   var prefs = Components.classes[ "@mozilla.org/preferences-service;1" ] .getService() .QueryInterface( Components.interfaces.nsIPrefBranch );
   prefs.setBoolPref( "network.protocol-handler.external.shell", false );
   prefs.getBoolPref( "network.protocol-handler.external.shell" );
   

Note:

1. You shouldn't need to restart, it will be saved the next time you do, but the effect is immediate.
2. There are only 3 lines above, slashdot breaks the first line apart.
3. You should see a false after evaluating the 3rd line.
4. If you run into any trouble, start again from above.
5. If you have trouble with the slashdot-munged code above, go to my blog and copy it from the textarea.