Hi! jacknicholson, welcome to 7f
Did some research, and I could not find anything on your subject either.
Best guess is no one at Mozilla ever thought that anyone would need to enter safe mode without making 'some' kind of change.
I even looked in, about
:config and could not find any entry that might relate to turning off and/or bypassing that options/dialog box.
Do not know how long it would take to get an answer, but you could always make a bug report at https://bugzilla.mozilla.org/