Linux

Boot with a live CD from a different distro. This will split a hardware issue from a software issue.

Check your storage connection! If storage disconnects, your OS will freeze and stop responding to the keyboard. Also, the os won't be able to write any logs because the storage isn't attached. Even power off won't work because the os can't read any files. This feels very similar to your problem.

For me, my motherboard had a faulty drive controller which would randomly stop working and drives would no longer appear connected.

I'm not sure whether you have the same issue as me but it has the same characteristics as mine. Hope this helps!

I was in a similar situation not too long ago and couldn't find anything to fix it either at first. One thing that was high on my list was changing my PSU since a defect or weak one often seems to be a problem in such cases. Besides a general hardware failure of course. If it's the hardware that could be anything really. Motherboard, RAM, GPU, PSU. PSU is the easiest to switch tho, so if you go that route I would try that first.

Anyways, I never had to do this cause in my case, believe it or not, a BIO update fixed my problem. I am still not 100% sure what happened but I think the update fixed the GPU voltage distribution or something similar.

Hope that help at least a little bit.

Test your RAM. I had a machine doing this a few years ago - turns out I had a stick of RAM with a 128k block somewhere in the middle that was dead.

That machine worked fine as long as I didn't get it doing anything too intensive, then it would crash. A new stick of RAM solved the issue.

If you have another pc, ssh from it to the problem machine and run sudo dmesg -w. That should show kernel messages as they are generated and won’t rely on them being written to disk.

Can you be more specific when you say "plays videos"?

Like in vlc, or YouTube, or something else? What videos? Like, 4k hevc videos, or literally anything?

Try this

https://memtest.org/

Why do you suspect gfx card?

What happens if you use a different video player, or play different videos ie different codecs?

I used to have a similar issue but updates at some point fixed it. I think it was codecs.

Do you have an onboard gfx card you can use instead to test?

I think a live boot cd or trying to use an integrated gpu, if available, (both I saw already suggested) are better steps but you could also try blacklisting nvidia and use nouveau. Could point to those drivers if it works ok.

journalctl helps with viewing log files.

I had a similar problem a while back and it turned out to be my Asus motherboard's "AI" frequency control hard locking the system. Took me days of troubleshooting and headaches to figure this out. Ended up switching it off in BIOS and everything is stable now. Just my 2c.

Would it be too much to remove the GPU and run videos? How often does it happen? If it's easy to reproduce, and it's not too much work, you can try removing the GPU and using onboard gfx to see if the problem persists.

Another suggestion, maybe try different drivers.

Alternatively, you could boot do a new install to a usb, install the same or different drivers or both, to see if the problem persists.

This does seem like a stability issue, either on the hardware or firmware side. It could even be as simple as reseating the GPU.

btw, do you happen to remember whenabout the first crash happened? Did it start out sporadic and grow more frequent?

I read you mentioned firefox. I had a similar experience a while ago, related to this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1704774#c13

I used to have some similar issues when playing games, and the cause of it was my motherboard's firmware. Maybe check and see if it is up to date?