Yes, it's a tricky problem because "what does autoplay mean?" If you load up a page containing an HTML5 game, presumably you want the background music and SFX to work. But how do we differentiate those from an unwanted autoplaying audio ad?

I believe the Firefox devs settled on anything during/after a user interaction is allowed to autoplay. More at this bug (which I've been following for years, if you couldn't tell :)

https://bugzilla.mozilla.org/show_bug.cgi?id=659285