The only explanation is that they don't give a crap. Very frustrating.
Reading the bug thread cited above, it may even be a case of, "I was able to get Chrome to crash under similar circumstances, therefore it is conforming behavior. WONTFIX."
What platform are you using? I have used FF exclusively for 10+ years on Linux. Ubuntu and OpenSUSE every day. Nouveau and i915 every day. If I said I get 3 crashes per year in the last 3-5 years that would probably be exaggerating.
The only time FF 'crashes' is when I update while it's running, and even then it gracefully degrades and lets me know I have to restart it. With the 'restore previous session' menu option (an option that only exists in piecemeal and implemented much worse in Chrome), this is a mild inconvenience.
Chromium crashes much more frequently for me on Linux.
https://bugzilla.mozilla.org/show_bug.cgi?id=1210727 - 4 years old
https://bugzilla.mozilla.org/show_bug.cgi?id=1010527 - 6 years old
Not to mention the weird random full-blown crashes that occur when I try to use it that I can't reproduce, no point in opening an issue for those.