I accidentally entered full screen view and was unable to exit. The Firefox Help article at https://support.mozilla.org/en-US/kb/how-make-firefox-and-websites-go-full-screen?esab=a&s=full+screen+mac&r=0&as=s says I should see a Full Screen View icon in the upper right-hand corner of the Firefox window. That icon not there. I cannot find it. It does not appear in either the full screen or non-full screen views. Further, the Help article's advice to exit full screen view by using the keyboard sequence command+shift+F does not work. I had to unplug my iMac and reboot. Firefox then opened in non-full screen view. Ideally, hitting the Escape key would cause Firefox to exit full screen view.