So I did solve the apparmor problem (not even sure how, some things I did were:
sudo /usr/bin/bwrap --ro-bind /usr /usr --symlink usr/lib /lib64 --ro-bind /etc /etc --dir /var --dir /run --dev /dev --unshare-ipc --unshare-pid --unshare-cgroup --unshare-uts --hostname unbound /usr/bin/unbound -d
sudo /usr/bin/bwrap --bind / / --dev /dev --unshare-ipc --unshare-cgroup --unshare-uts --hostname dhcpcd /usr/bin/dhcpcd -q -b
flatpak override --reset org.mozilla.firefox
and uninstalling flatpak-kcm but I don't know what exactly did the trick.)
However the problem of Firefox not being able to download anything still persist, so the apparmor was not the underlying cause. Anyone has an idea what might be behind that?