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 half-second clip 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 completely, instantly.
My Ubuntu 24.04 is on an NVMe drive instead of a HDD, and it’s the only OS on the drive. It’s using a standard ext4 filesystem, so it should be (and so far has been) safe from getting corrupted by the abrupt shutdown.
I have done one pass of memtest to rule out memory errors.
My system doesn’t have Nvidia; instead 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.)