Comment 2 for bug 231421

Here is a workaround, but someone should take the not working plugins out of firefox and add this plugin to the list:

http://ubuntuforums.org/showthread.php?t=229671

HowTo: playing embedded midi in firefox
Info
Midi files aren't working in firefox without a special plugin and ofcourse a midi software 'renderer'.
The plugin doesn't cooperate very nice with Acrobat Reader in firefox, but using xpdf in firefox solves this problem.

Installation
Open up a terminal and execute
Code:

sudo apt-get install timidity mozplugger

Answer yes to all questions and the installation will be complete.

Restart firefox (don't forget to close all instances) and voila, embedded midi files should now play.