I have "MIDI Sequence" on my Firefox Options tab, but it seems to have a nonsense content type. (To view the associated content type, I chose Use other... and opened the application picker dialog. Screen shot attached.)

If the site identifies the midi file with the audio/mid content type, then it should play automatically in the QuickTime plugin (this is based on opening the page about:plugins and searching for midi on that page). However, this will not happen if the site uses some other content type, or if it is a forced download.

Can you link to a page with the midi files that have this problem (assuming no login is required)?

It's not just one single MIDI file, it's literally every single MIDI file and MIDI player. Quicktime is fine with loading everything else though.

Sometimes the file that stores those Application Options becomes corrupted with double-entries and can't be fixed through the dialog. You can rename the file and let Firefox rebuild it.

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button. A new window should appear listing your settings files.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old

Start Firefox back up again. Any improvement in how midi files play?