Firefox Quantum: drop down menus won't stay down with single click
I have a Mac Mini running OS X 11.13.2. Since I installed Firefox Quantum (now 57.0.4) I've had this problem. When I single click on a drop down menu in a web form, the menu doesn't stay down. Sometimes if I keep clicking on the drop-down menu where there's no "arrow" it may stay down, but it requires as many as 6 or 7 clicks if it works at all. I haven't been able to find any references to this problem by doing a web search.
I've tried removing all the add-ons, cleared the application caches, deleted all the preferences and re-installed the software, all to no avail.
I'm able to select a menu item if I click and hold, but I can't make the menus stay down.
Does anyone know of a fix?
Thanks in advance...