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.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-26953
• https://bugzilla.mozilla.org/show_bug.cgi?id=1656741
• https://www.mozilla.org/security/advisories/mfsa2020-50/
• https://www.mozilla.org/security/advisories/mfsa2020-51/
• https://www.mozilla.org/security/advisories/mfsa2020-52/