Hello again Fred. Windows safe mode was not a winner and the same issue was present. Now I've found another situation. I have another computer in another room also running Windows 10 Pro and I updated its Firefox to 56.0. The problem is not present on that computer and Firefox closes normally with no crash report. Both computers are nearly identical except for some hardware so I can't begin to guess what might be causing the issue on this computer that I use most of the time.

I did a complete uninstall of Firefox and installed a new download. Looks like the problem is solved as I've closed Firefox and it does not show in the background apps. Must have been a corruption running the update. Problem solved. Thanks for the help Fred.

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.