Firefox

There are of course a myriad of reasons for which you'll get a lot of ideas. Some have already suggested running memtest.

I'd suggest a full hardware "scrub". Open the case, clean out all dust with a blower (don't forget the PSU!!!), wiggle or remove-reinsert all electrical connections, especially CPU and RAM, but also GPU and expansion cards, storage (data and power), etc etc. Basically if you can plug it in, give it a wiggle to break any oxide layers or as I said remove-reinsert. Might not even hurt to double check your CPU and GPU thermal paste contact if it's old and might have dried out.

Then double check your overall air flow. Your CPU might be cool as a cucumber, but maybe your m/b is getting too hot from lack of air flow. Ironically its possible that a cool CPU = slower fan = less airflow around SB chips and power chips.

I was having escalating browser freezes from random weekly to random daily to random hourly or worse, on a system that was stable for years. I did the above and added another case fan for better MB flow. Now my CPU runs cooler, the CPU fan runs even slower, and I've been rock steady for 2 weeks without a single freeze-up.