How to play a midi file in Firefox?

I know this isn't the answer you want, but modern browsers don't play midi files. To the best of my knowledge Mozilla dropped support for NPAPI plugins around the first quarter of 2017. Wikipedia makes this clear.

Music files may occasionally use the MIDI format (.MID or .MIDI extension). MIDI is often playable without additional software, and some PCs have a MIDI-enabled player and sound card. However, for macOS (since 10.9 Mavericks), a MIDI file is not playable by both Safari and QuickTime pre-installed in the system and an alternative standalone player has to be used. The situation is similar with modern Firefox and Google Chrome browsers which do not support MIDI by default but require additional software (plugins, extensions, codecs) to play such files.

One solution is to download and play them in totem (a.k.a Videos) Totem requires a plugin for this that it doesn't automatically find. The solution to THAT is to be found in the fluidsynth package.

Another option mentioned by doug in a comment above can be found here: Audacious: How to play midi files?

S number of popular MIDI files can be found here.

Sources:

https://support.mozilla.org/en-US/questions/1149132

https://en.wikipedia.org/wiki/Wikipedia:Media_help