Honestly, I think it’s great: if it annoys people enough, they will move out from Chrome and install Firefox (or one of its forks) with good extensions for blocking ads on YouTube. It seems to be the only solution to break the monopoly of Google on browsers. So go Google, I believe in you!

Pop!OS is a pretty stable distro, so there shouldn’t be any dependencies problem: you usually have that on arch or arch based distros, even if it’s quite rare. Have you ever uninstalled Firefox and reinstalled it but without some of its dependencies? Or ignored a package it asked you to install with it afterwards? If not, maybe it’s just an update that went wrong, like the PC shut down during an update or something. So update your system, uninstall firefox and reinstall. Your data stored in your home directory (in .mozilla, a hidden folder) like your history, extensions, opened tabs, should be conserved, even if you use apt purge.

Unless it’s a RAM filling issue, because you would have too little RAM, and even then, the entire OS crashing and not just Firefox would be very unlikely, I don’t see any reason for it to crash your entire OS. Regardless, there are Firefox forks that use much much less RAM. You’re using Windows, Mac or Linux first? Try to uninstall, clean everything and reinstall with a new profile, it may work, who knows. If not, then it has to be your OS.

You have to give them a reason to get interested in the OS and the programs they’re using. I gave Linux a try because I was concerned about privacy and I wanted to use more ethical and user respecting OS and software than what I used at that time. Linux and the FOSS world was an obvious choice for me. Custom ROM on Android was sort of the bridge which allowed me to transition. If it wasn’t for that, I would still be on Windows and I wouldn’t learn that much on how an operating system works and what differentiate them, aside from the look. The fact they’re kids or that they play games have nothing to do with it: a lot of adults don’t know either what type of OS they’re using, despite it being in their best interest. The problem is that we don’t give or show them the reason they should be interested, or at least be curious about it and most of time, before people get a degree, we end up killing their curiosity.

As they play Minecraft, you can advise them to switch to Prism Launcher instead of the minecraft launcher, especially if they mod the game, it’s much better for that. It could be a good start.