Cool, I’m not going crazy! I’ve had a very, very similar issue for a couple of weeks now. Identical in fact, except for the triggers and frequency: three incidents so far, since the first one on January 11th, with the last two within two days earlier this week. In all instances I’ve had Twitch playing video in Librewolf, when suddenly the desktop freezes, leaving only a 500 ms bit of the audio looping endlessly. I had forgotten about magic SysRq, so I have yet to try if it works; I’ve also just done a hard reset instead.

If the logs posted by anonymousdormouse are related to the issue, then that’s one difference wrt. what’s happening here: there’s been nothing related to the problem in any logs. The system seems to just die instantly and completely.

Ubuntu is on an NVMe drive instead of a HDD, and it’s the only OS on the drive.

I have done one pass of memtest to rule out memory errors.

My system doesn’t have Nvidia, it’s using the integrated GPU of a Core i7-8700 with two external displays (one DP-connected, one HDMI-). (This is a Dell XPS desktop that originally came with an external Nvidia GPU, but I ripped it right out before installing the system. I’ve had enough bad experiences with Nvidia to know to avoid them whenever possible.)