Did you migrate over from Arch or something? Because I install via the iso image. I don't see any /etc/firejail or firejail anywhere here. Please provide more information concerning your issue.

this is essentially the same issue as with bleachbit - those files are nothing to worry about - they are not executable code, and they are freely licensed files; so there is no freedom or privacy problem with them - the only reason to remove them would be if the program suggests using the other non-free programs

FWIW, you could make your own customized package which removes them - we do like to encourage poeple to learn how to use makepkg to build their own packages - if you need help with that, just ask

stretchp - the LiveISOs do not have firejail installed - you could use the pkgfile program though, to show which files any package will install

$ pkgfile -l firejail

How can I cross-compile it for i686 and arm from x86_64?

If it requires a VM, I don't have hardware virtualization support.

libretools lets you do this. i686 doesn't require a vm are requires binfmt-qemu-static Not sure if you'd consider that a vm or not.
See the info on building for ARM for more details hete:
https://wiki.parabola.nu/How_to_build_ARMv7_packages_from_a_chroot_with_PRoot_for_Parabola_ARM