Firefox behaved slightly differently for already known resources when loading CSS resources involving CSS variables. This could have been used to probe the browser history.
An improper implementation of the new iframe sandbox keyword allow-top-navigation-by-user-activation could lead to script execution without allow-scripts being present.
Mozilla developers Andrew McCreight, Gabriele Svelto, Tom Ritter and the Mozilla Fuzzing Team reported memory safety bugs present in Firefox 99 and Firefox ESR 91.8. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.
When reusing existing popups Firefox would have allowed them to cover the fullscreen notification UI, which could have enabled browser spoofing attacks.
Documents in deeply-nested cross-origin browsing contexts could have obtained permissions granted to the top-level origin, bypassing the existing prompt and wrongfully inheriting the top-level permissions.
Mozilla developers Gabriele Svelto, Randell Jesup and the Mozilla Fuzzing Team reported memory safety bugs present in Firefox 99. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.