A workaround is to open Firefox's Preferences, select Applications and change the Action for the "MIDI audio" Content Type to "Always ask", which will restore the original behaviour, or "Use Movie Player (default)" which is slightly more convenient than "Always ask".
A workaround is to open Firefox's Preferences, select Applications and change the Action for the "MIDI audio" Content Type to "Always ask", which will restore the original behaviour, or "Use Movie Player (default)" which is slightly more convenient than "Always ask".