All I can tell you is that Firefox has been rock solid stable for all the years and versions I've been using it.
What follows is not aimed personally, but it is a general observation: Virtually always these "weird issues" and "odd problems" stem directly from operator error of some sort. The trick is figuring out what that operator error happens to be.
I've been around this block more times than I care to count with more types of software I care to count. While bugs can and do account for some small fraction of "weird errors" it is a small fraction.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore