alt Hacker NewsFirejail uses "Linux container" technology (term?) which is not that secure. Better is using selinux to confine the browser, like Android and ChromeOS do.
(Fedora and Red Hat have selinux, too, but the focus is on server security: there is no attempt to confine browsers in the selinux rules that ship with Fedora and Red Hat.)
For me the interesting part of Firejail is the interface. bwrap is usually recommended as a replacement given that the binary is smaller and thus offers less attack surface, which I think is the usual concern. Firejail employs kernel user_namespaces, but also offers integration with AppArmor.