If a document created a sandboxed iframe without <code>allow-scripts</code>, and subsequently appended an element to the iframe's document that e.g. had a JavaScript event handler - the event handler would have run despite the iframe's sandbox. This vulnerability affects Firefox < 97, Thunderbird < 91.6, and Firefox ESR < 91.6.
https://bugzilla.mozilla.org/show_bug.cgi?id=1739957
https://www.mozilla.org/security/advisories/mfsa2022-04/