After six months of frustrating system crashes every time I started Firefox in normal mode, and it taking several hours to recover my system after each crash, I think I finally got to the bottom of this, aka, the root cause of the crashes. I'm still not entirely sure why the crashes only happened when Firefox is started - I did not have this experience with any other applications on my system.
In August I replaced my hard drive and the hard drive cable that connects it to the logic board, installed a fresh version of the OS, and restored all my data and applications. For Firefox, I also had to restore my bookmarks from a backup since those disappeared with the update to 69.0. After all that, the system crashes continued every time I started Firefox in normal mode. When I started Firefox in safe mode, the crashes did not occur.
After lots and lots of research I learned that the symptoms I was experiencing may be due to a failing GPU. I found instructions for reflowing the GPU and took care of that. Since completion of that little task, Firefox starts in normal mode without issue. Today I updated to 69.01 and started it back up in normal mode. Bookmarks are still present and Firefox doesn't crash my system.
We can now call this resolved.
Who'd a thunk, right?
Have a good day,