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.
You can download an updated version of Mull with the security vulnerability fixed, from the DivestOS F-Droid repository: https://divestos.org/fdroid/official/. If you currently have the F-Droid version of Mull installed you will need to uninstall it first.