To fix the "MMS is not a registered protocol" message when trying to view streaming content, here's the fix:
(1.) Open Firefox, type "about:config" (no quotes) in the address window, and click enter.
(2.) Right click on the window and choose [New], then [String] from the pop-up menu that appears.
(3.) In the first pop-up box, enter: "network.protocol-handler.app.mms" (no quotes, and it might just be easier to cut 'n paste this into the box).
(4.) In the next pop-up box enter the path to Kaffeine (e.g. "/usr/bin/kaffeine").
(5.) Now, click in the main window again but choose [New] [Boolean]
(6.) In the first pop-up box, enter: "network.protocol-handler.external.mms"
(7.) In the second pop-up select [True].
AS AN ALTERNATIVE!!!!
Install the "media player connectivity" extension for Firefox from the url below, which allows you to open all embedded audio/video links in external media programs.