My first instinct is that you're looking too deep. Clean the crap out of your cooler fins & fan grills. Maybe take off a side panel. Sounds like overheating to me.
"We all have flaws, and mine is being wicked." James Thurber, The Thirteen Clocks 1951
Clean boot and (apparently) shutdown this morning. My suspicion now points to a FF memory leak maxing out the swap file over time, preventing a clean shutdown or (eventually) causing a hard crash. xp appears to be unaffected and has carried on as per usual.
“Former US Senate candidate and first Maine resident to catch COVID Max Linn found dead in hot tub after being sued for pulling gun on former aide during cryptocurrency disagreement”