It is hard to judge, but a crash on exit seems to me a possible consequence of a damaged memory. Firefox frees all the resources and collects the garbage. I expect it to touch a lot of memory locations, and do something with values retrieved.

> this crash-on-exit happens due to many different causes, as seen in the crash reports

It points to the same direction: all these different causes are just symptoms, the root cause is hiding deeper, and it is triggered by the firefox stopping.

It is all is not a guarantee that the root cause is bitflips, but you can rule it out by testing your memory.