You are mostly wrong about everything and if you tried to solve the problems instead of blaming the devs you'd be better off. If these were browser problems they would be happening to everyone. I sit here with older slower hardware and have 0% cpu in firefox and do anything I want, you guys can't even check protonmail and you blame it on firefox.
Downloading is the failsafe method. streaming is just more problems and more power. I'm not sure how you missed this basic fact since streaming came long after downloading. You're also wrong about what's happening when you download things, TCP IP and HTTP don't work that way, when you download a mp4 you get a mp4 only. there is no way to slip in extra bytes.
You can't figure out VLC ? Yikes,,, you mean like click the icon which is all VLC requires, its a player not CAD .. there is nothing to figure out besides play and stop . If you did this you would at least know where your probelem is.
Firefox is a crappy company but rendering pages is a pretty much foolproof and this is the one thing they are good at.
With XP and the age yes we do have to point to hardware problems. Most people are running on 15 year old hardware so expect problems and expect them to show up where max power is used. With youtube you are using sound, video, networking so exepct this to be the place where the problem pops up but isn't always the source of the problem, you are never going to crash in notepad that ues 50k memory and no cpu.
Why don't you download something, play it locally and report what happens.
https://freeyoutubedownloader.online/