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.)

After updating Firefox from 78 to 79, hardware-accelerated video decoding no longer works properly in Wayland: streaming video gets cut off with an error message from the service.

== Steps to reproduce ==
1. Follow the steps in [1] to enable VA-API.
2. Open a Youtube/Twitch video (for instance https://www.twitch.tv/rifftrax), press play

== What I expect to happen ==
For the video be played without issues (as it did with Firefox 78).

== What happens ==
After playing for a while (anything from just a few seconds to a couple of minutes), the video stops and is replaced by a service-specific error message, such as error #3000 (in case of Twitch) or (Youtube). Even when playing, the video also flickers green. After reloading the tab, the video again plays for a few seconds before the error message reappears.

== Other info ==
A fix is apparently already in the works upstream [2].

* [1] https://mastransky.wordpress.com/2020/06/03/firefox-on-fedora-finally-gets-va-api-on-wayland/
* [2] https://bugzilla.mozilla.org/show_bug.cgi?id=1645671

With `media.autoplay.enabled.user-gestures-needed` set to false and `media.autoplay.default` set to 1 (block audio), which is the default (and other autoplay-related settings in their defaults as well), a Twitch video can only be either paused or playing muted; unmuting the video pauses it and vice versa.

== Steps to reproduce ==
1. Open about:config
2. Search for autoplay in the settings
3. Set media.autoplay.enabled.user-gestures-needed to false
4. Set media.autoplay.default to 1
5. Open a Twitch VOD, for instance https://www.twitch.tv/videos/280106033
6. Try to unmute the video

== What happens ==
The video is unmuted, but also paused

== What I expect to happen ==
For the video to continue playing, unmuted

I’m able to reproduce this when I set media.autoplay.enabled.user-gestures-needed to false; if set to true (which is the default), unmuting does not pause the video.

The Report button doesn’t seem to do anything for the one ad I’ve reported dozens of times by now: please do something about the obnoxiously loud (Finnish) Weekend Festival ad. Like I said, I’ve already reported it as such countless times using the report button, but Twitch still keeps playing it to me.

I don’t mind ads per se, but I’m now seriously contemplating installing an adblocker just because of this one ad.