When reusing existing popups Firefox would have allowed them to cover the fullscreen notification UI, which could have enabled browser spoofing attacks. This vulnerability affects Thunderbird < 91.9, Firefox ESR < 91.9, and Firefox < 100.
https://bugzilla.mozilla.org/show_bug.cgi?id=1746448
https://www.mozilla.org/security/advisories/mfsa2022-16/