(Removing question -- I have resolved it)

(Sorry if I wasted anyone's time. The issue turned out to be my large external monitor for one of my computers. When I was not using it, the smaller laptop screen did not allow all Firefox setting pop-up windows to display fully, so I could not see what I was used to seeing. When I connected it again to my large external monitor, everything was fine. That is why the other computer had no issues.

(So it had nothing to do with Firefox at all, and I am sorry if anyone read my question! Thanks.)