i have the exact same setup on ECC ram and zero crashes. on non ECC (cheap, garbage, that everyone accepted as the default) ram, one crash every couple weeks.
so unless you can prove software on the same cpu is non deterministic, it is ram corruption.
I've been using Firefox on Ubuntu since 18.04 was first released (about 6 years ago), and while I've encountered some issues, I haven't experienced the problem you're describing.
Of course, browser performance can be affected by many factors in your system. If you're seeking help, you might have better luck in a dedicated support forum or the official Firefox support channels.
In any case, I've been been a Ubuntu user since ~2010 (and a Firefox user since its inception). I remember there being a time when Firefox was slower than Chrome and freezing occasionally but that was a looong time ago and I haven't had any issues with performance or freezes/crashes ever since.