These instructions will configure Firefox to use Gmail or your Google Apps hosting account to handle mailto links:
First, type about:config in the address bar, search for gecko.handlerService.allowRegisterFromDifferentHost, and set the value to true.
Then, log into GMail or your Google Apps mail URL, paste this line of JavaScript into your address bar, and hit enter:
For Gmail:
javascript:window.navigator.registerProtocolHandler(”mailto”,”https://mail.google.com/mail/?extsrc=mailto&url=%s”,”GMail”)
For [...]

Here is how to control which application Firefox uses to handle MMS streams:
Type about:config Firefox address bar.
Right-click on any text in there and select “New” and then “String”.
In the first dialog box, as preference name, type network.protocol-handler.app.mms.
In the second dialog box, as string value, type in vlc (or the path to your favorite media player)