The fullscreen notification is prematurely hidden when fullscreen is re-requested quickly by the user. This could have been leveraged to perform a potential spoofing attack. This vulnerability was fixed in Firefox 135 and Thunderbird 135.
Exploitability
AV:NAC:LPR:NUI:NScope
S:UImpact
C:NI:LA:N5.3/CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N