I din't know that could be done Dave (and have Firefox to prompt with Rosegarden as an option).   I've just tried timidity commands using urls as per the manual and all I get is file not found.

I suppose I could use pmidi to open links in Firefox. I don't see how you get the parameters in and have them work but a simple script like:

  #!/bin/bash
  pmidi -p 128:0 $1

works. The drawback is you have to play the whole file (or kill the process).

Years back, I had a setup, possibly using mozplugger???, but I don't remember what happened there...