Bug report is here - https://bugzilla.mozilla.org/show_bug.cgi?id=911837 - It's been a while since I looked into it and looks like the details have changed recently. At least a quick attempt now tells me "Please refresh this page in your browser to get back to listening to your music. If issues persist, make sure you have Flash enabled and working."
By which I mean, a standard that works cross-browser? ;)
YouTube encodes all their content with in multiple formats: AAC and Opus, H.264 and VP9. I don't know why Google Play Music can't encode their music streams in both MP3 and Opus. They would save on bandwidth by using Opus.
Here is the Firefox bug to add support for MP3 in MSE: https://bugzilla.mozilla.org/show_bug.cgi?id=1169485
There are game engines now that can compile to a pure HTML / css/ Javascript runtime.
Flash is a known attack vector. I'd be more comfortable with the argument "Kids should be free to play games" if I didn't have to keep purging virii off my aunt's computer because she won't stop visiting Flash game sites.
Besides, there are alternative browsers. If kids need to drop into Edge to play Flash games, that seems like an acceptable solution? If it isn't possible (because they're on a school network that whitelists browsers, for example), that's indicative of the cost / benefit tradeoff of that use case.