If a user downloaded a file lacking an extension on Windows, and then "Open"-ed it from the downloads panel, if there was an executable file in the downloads directory with the same name but with an executable extension (such as .bat or .exe) that executable would have been launched instead. *Note: This issue only affected Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

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.

Using techniques that built on the slipstream research, a malicious webpage could have exposed both an internal network's hosts as well as services running on the user's local machine. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

When flex-basis was used on a table wrapper, a StyleGenericFlexBasis object could have been incorrectly cast to the wrong type. This resulted in a heap user-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

Certain input to the CSS Sanitizer confused it, resulting in incorrect components being removed. This could have been used as a sanitizer bypass. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

Certain blit values provided by the user were not properly constrained leading to a heap buffer overflow on some video drivers. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.