Sorry, I have FF version 57.0.2 (64 bit)

From what you and Ken say, it may be the new version
of FF that may be causing this intermittent problem.

I checked the last comment on the link and I'm very
iffy about going into about:config because I may screw
things up royally.

I tried going into Safe Mode but it never did. I pressed
F8 repeatedly at intervals and the MBR appears but nothing
happens. I tried this several times.

Thoughts/suggestions,
Robert