There was a critical vulnerability found on Firefox some days ago: CVE-2024-9680. Fennec and Mull are forks of Firefox. They both fixed this issue already in their source code, BUT there is a problem preventing F-Droid from building these updated, fixed versions.

In the case of Mull, you can download the updated version from the DivestOS F-Droid repository: https://divestos.org/fdroid/official/, but if you are currently using the F-Droid version you will need to uninstall it first, since they have different signatures.