To have Firefox look a little more at home on macOS, check out the following addons:
I've also noticed that Firefox 50.0.2 sometimes crashes when disconnecting the external monitor from my 2016 MacBook Pro. After restarting, it brings up the Restore Session screen with the "Well this is embarrassing" message. To turn this off, go to about:config, search for resume_from and double-click on the value browser.sessionstore.resume_from_crash so it's set to false.