tl;dr: add the
non-free component to the line that defines the main Debian repository in your
linux-firmware-nonfree is in the
non-free Debian component, so you should find the line in
/etc/apt/sources.list that looks like
deb http://<some url here>/debian <distribution version> main [contrib], and add
non-free somewhere after
main. Then, to install, run
sudo apt-get update; sudo apt-get install linux-firmware-nonfree.
If you are using oldstable or stable, you should also add
non-free to the
security.debian.org lines, if you have them enabled. If you like, you can do the same to both
deb-src lines, but it doesn't really matter.