When a user clicked on an FTP URL containing encoded newline characters (%0A and %0D), the newlines would have been interpreted as such and allowed arbitrary commands to be sent to the FTP server. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.
https://www.mozilla.org/security/advisories/mfsa2021-16/
https://www.mozilla.org/security/advisories/mfsa2021-15/