If a user clicked into a specifically crafted PDF, the PDF reader could be confused into leaking cross-origin information, when said information is served as chunked data. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.
https://bugzilla.mozilla.org/show_bug.cgi?id=1683940
https://www.mozilla.org/security/advisories/mfsa2021-03/