A flaw during verification of certain S/MIME signatures causes emails to be shown in Thunderbird as having a valid digital signature, even if the shown message contents aren't covered by the signature. The flaw allows an attacker to reuse a valid S/MIME signature to craft an email message with arbitrary content.
A buffer overflow vulnerability in the Skia library can occur with Canvas 2D acceleration on macOS. This issue was addressed by disabling Canvas 2D acceleration in Firefox ESR. *Note: this does not affect other versions and platforms where Canvas 2D acceleration is already disabled by default.*