Keyboard events reference strings like "KeyA" that were at fixed, known, and widely-spread addresses. Cache-based timing attacks such as Prime+Probe could have possibly figured out which keys were being pressed. This vulnerability affects Firefox ESR < 102.5, Thunderbird < 102.5, and Firefox < 107.
https://bugzilla.mozilla.org/show_bug.cgi?id=1793676
https://www.mozilla.org/security/advisories/mfsa2022-49/