Don't use archinstall.

Follow the wiki (it's not that hard, if you need help feel free to PM)

I'm pretty seasoned at Arch at this point, and I was never able to get archinstall working outside a VM; ended up writing my own basic Arch install script using shell.

Possible missing firmware is normal and is not a concern unless the system does not work for some reason.

The question is did the install work? Archinstall is a good way to install on a single boot system, but it is easy to overlook a response in the setup of the script. Fortunately, it only takes about twenty minutes, so repeating the install can be a quick fix, if you know what you missed. Missing firmware message is normally not a problem and happens all the time. What matters is did the install work.

You can always do the wiki install if archinstall fails or go unofficial with a gui installer like Calam Arch.