When an extension with the proxy permission registered to receive <all_urls>, the proxy.onRequest callback was not triggered for view-source URLs. While web content cannot navigate to such URLs, a user opening View Source could have inadvertently leaked their IP address. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.
https://bugzilla.mozilla.org/show_bug.cgi?id=1657916
https://www.mozilla.org/security/advisories/mfsa2020-54/