I guess i need something more lightweight than Qubes.
Is it a good enough alternative to just run all untrusted applications using Firejail?
Good enough always depends on the value of what you are trying to protect, the likelihood of a compromise and your risk tolerance. What may be good enough for you, may not be for everyone else using Qubes 
As a ‘layman’ I’m under the impression that Firejail is SIGNIFICANTLY less secure than Qubes OS. You’re dependent on the security of the Linux kernel, which the Qubes devs have stated they have no confidence in(otherwise why would they develop Qubes OS ;)) Compared to Xen, there is a huge attack surface, which makes it a lot easier to bypass compartmentalization compared to that offered by Qubes. And Qubes adds additional security as well, for example with isolation network and usb controllers through IOMMU. However, using a standard Linux distribution with Firejail is of course more secure than without Firejail. It all depends on your needs and threat model.