It was possible to cause the browser to enter fullscreen mode without displaying the security UI; thus making it possible to attempt a phishing attack or otherwise confuse the user. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
https://www.mozilla.org/security/advisories/mfsa2020-52/
https://www.mozilla.org/security/advisories/mfsa2020-51/