Edite /etc/firejail/firefox.profile e comente ou exclua a noroot line
Como posso executar o firejail como usuário root?
# firejail --seccomp firefox
Reading profile /etc/firejail/firefox.profile
Reading profile /etc/firejail/disable-common.inc
Reading profile /etc/firejail/disable-programs.inc
Reading profile /etc/firejail/disable-devel.inc
Error: --noroot option cannot be used when starting the sandbox as root.