These days, new AudioContext() and requestMIDIAccess() need to be initiated by a user gesture.

Error messages in Firefox:

An AudioContext was prevented from starting automatically. It must be created or resumed after a user gesture on the page.

Uncaught (in promise) TypeError: selectMIDI is null
    populateMIDIInSelect https://webaudiodemos.appspot.com/midi-synth/js/midi.js:44
    onMIDIStarted https://webaudiodemos.appspot.com/midi-synth/js/midi.js:87