Maybe this project will expand in the future to other popular distributions, including Arch.
Very unlikely. Not only is Arch behind Fedora as a base on which to build a hardened image, it lacks the breadth and depth of tooling and surrounding ecosystem for image building (ublue, blue-build, rpm-ostree, etc)
How should secureblue users install Firefox, Mullvad Browser, Brave Browser, Tor Browser, etc., should users follow the steps here ?
It’s not recommended that you use Firefox based browsers (for security reasons), but if you need them for some reason then yes the link you sent is correct (except for chromium based browsers like brave, which should never be installed via flatpak since flatpak interferes with the much more robust internal chromium sandboxing: Flatpak support | Vivaldi Forum)