Alas, I am not a web wizard, but your query got me curious.
It appears Firefox 66.0 and up blocks auto play by default to avoid the well known jolt of audio blasting out when you enter a new page. An article here:
https://hacks.mozilla.org/2019/02/firefox-66-to-block-automatically-playing-audible-video-and-audio/ So that's one change. Purportedly it will auto play if the volume is muted (which doesn't accomplish your goal!)
You can configure exceptions to that on a URL basis in the browser options. I used "view source" to see what you were installing and was able to get a small local HTML file to auto play here (Firefox 66.0.4) by configuring an exception for my local file. Under those conditions, a typical mini-player shows up with a play/pause button, elapsed time bar, and volume control. Before I configured my browser to allow auto play, the player appeared, but I had to click the play button for it to run.
But even with the exception set for the URL of your example PBase image in my browser I don't see that player -- or hear music -- did that player used to show when this stuff worked? Mayhaps it's an unholy combination of browser changes and the "New Hardware" at PBase.
Good luck (and I like the music!)
DaveT