Firejail is a SUID sandbox program that reduces the risk of security breaches by restricting the running environment of untrusted applications using Linux namespaces, seccomp-bpf and Linux capabilities.
Questionable whether we really need this in Solus or not. You shouldn't be running untrusted applications on Solus in the first place and we wouldn't be providing via the repo "untrusted applications".
It would act more as a protection against zero-day vulnerabilities, and would have e.g. helped against this Firefox bug due to only letting Firefox access a few select folders.
Here is another example:
But yeah, I guess the general interest in this package would be pretty low so those interested can just compile it themselves.