Here is how to setup FireFox to open an external mail application ... and how to set it to be what you want:

1. Type "about:config" into the URL field in Firefox
2. Right click anywhere in the list of preferences
3. Choose New->String in the pop up menu
4. Type "network.protocol-handler.app.mailto" for the name
5. Type "/usr/bin/evolution" (or the path to the mail app you use) for
the value
6. Restart Firefox