After use and not even long use dropdown menus are blank, Firefox won't close.
For a couple of weeks after a short time of use dropdown menus are blank almost as if they have been lifted off the screen but I can see an imprint of where they should be.. Sometimes running the cursor over them seems to make them appear but not often. When this happens I cannot close Firefox.
Every time Firefox opened
== a couple of weeks ago
From your description, it seems that an add-on may me causing your issue. Follow the steps at Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems to test whether your extensions are causing this behavior. Reply back here with the results and with the name(s) of any extensions you have enabled.