firejail is just a management layer for the same things that docker uses. But with less holes. And the option to plug other features via apparmor. And yes you are required to tweak to get the right level of convenience you want. Docker is more popular and not many other merits besides that. I mean you can use firejail without tweaking and just allowing all the things docker offer by default to get the same false sense of safety, but you already have docker for that.
