I haven't been able to find much information on kiosk mode but from what I can tell it is exactly that: a mode designed to run Chromium for public kiosks/terminals, normally intended to run a single website or web app. In other words, start the browser automatically, run full screen and hide all controls (buttons, menus, etc) from the user. Any additional security layers that it provides would be to protect from someone with physical access to the device and it makes little sense to use it for general web browsing on a device you own. There might be a slight memory advantage as it doesn't load user controls.

For something like Chromium, I probably wouldn't bother using Firejail. The Chromium developers are very hot on security and it's already heavily sandboxed. They already do system call filtering with seccomp-bpf (if the kernel provides it) and I'm pretty sure they already drop unneeded capabilities. Don't forget that excessive sandboxing can break stuff. Google know their product better than the Firejail developer so I would just trust them on this one.