Google in their wisdom had disabled the ability to be automatically logged out of Gmail accounts on browser shutdown. That is unless one blocks javascript on accounts.google.com on the gmail.com login page. If javascript is blocked then you should be presented with the older style login screens with actual data entry boxes. NoScript for Firefox, the newly released version does allow this to work. However with the newest Firefox ScriptSafe, if I deny or distrust accounts.google.com I can not login at all. I see no login screen.
Under "Disable and Remove" the is unchecked.
Other script blockers like uMatrix simply does not pass on the flag to Gmail so the result is that you still see the newer style login screen.
This seems to reflect the same issue as reported #136 which was for the Chrome version of ScriptSafe but that issue was Closed