How do I associate a new protocol with a program in firefox using linux?

I'm using arch linux, and I've tried a couple times to add new protocols to firefox but have failed, with firefox popping up with "Firefox doesn't know how to open this address, because the protocol (mms) isn't associated with any program."

For example I'm trying to get firefox to open mms with mplayer, and following the mozilla support page http://support.mozilla.com/en-US/kb/The+protocol+is+not+associated+with+any+program and other info found on the net added these two preferences to about:config:

network.protocol-handler.app.mms   /usr/bin/mplayer
network.protocol-handler.external.mms   true

It achieves nothing and I get the exact same error next time I click an mms link, even after restarting firefox. I can open the mms link from the command line with mplayer though. I has been bugging me for ages because it should be so simple but nothing I have found on the net will work.

By the way I want a general solution to this, not one just related to the mms protocol.

Cheers