...è un bug conosciuto di Firefox....

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.3) Gecko/20041019 Firefox/1.0
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7.3) Gecko/20041019 Firefox/1.0

If you have Windows Media set to play non windows media file types (such as mp3
or midi) then the plugin doesn't work in Firefox.

This is because pluginreg.dat only contains Windows Media Player entries for
windows media file types.

Reproducible: Always
Steps to Reproduce:
1. Ensure Windows Media Player is your default player for midi files
2. Check that Quicktime, and any other player, is not set to handle MIDI mime
types (easiest way is checking aboutlugins).
3. Load a web-site with an embedded MIDI file.

Actual Results:
It asks you to install QuickTime

Expected Results:
It should use the Windows Media Player plugin.

Changing the WMP section of pluginreg.dat to the following broadly fixes the
problem. I haven't included all the extensions handled by WMP, and have probably
some in there incorrectly but you get the idea.