They are completely different mechanism for doing different kind of stuff. Firejail sounds like something closer to Capsicum, but without the security model.
I was under the impression they were like jails, to sandbox a program and make it more siloed off and secure? Whatever the underlying mechanisms for obtaining that. I'll research some more I guess.
Could you elaborate on the differences? As far as I understood it firejail, or rather the Linux features that it depends upon, is far more powerful than FreeBSD jails.
From what I understand, firejail is a "syscall filter". This moves it to the same category as capsicum (https://www.freebsd.org/cgi/man.cgi?capsicum), but without Capsicum's security model, instead implementing something ad-hoc, probably by using Linux' seccmp.
Jails, on the other hand, are not a sandboxing mechanism - they are system-level virtualization, like Linux namespaces, but with a simpler interface. You can use it for sandboxing, but it's not what the mechanism fundamentally is.