Foreword: yeah, yeah, by now everyone should know the limits of F-Droid - and of FF browsers, yes - but in any case...
If you have Mull or Fennec installed from F-Droid, make sure you've got version 131.0.3 or newer. (settings > about)
As of today, both browsers are still at 129.0.2 in the official Fdroid repo. On top of the regular updates, this version lack this patch: https://thehackernews.com/2024/10/mozilla-warns-of-active-exploitation-in.html
So, if you have anything older than 131.0.2, back it up (sync, whatever), uninstall it and install the github version with Obtanium.
Or, ir you're dead set on using fdroid/NeoStore/etc, uninstall the fdroid version and install the one from the DivestOS repository.