Add programs.firefox.enable = true; to configuration.nix.

Call sudo nixos-rebuild switch. Maybe add --flake if you're feeling adventurous.

Done.

Or, more realistically...

programs = {
    firefox = {
        enable = true;
        # Configure anything you want here (language,
        # extensions, policies... whatever).
    };
    # Enable and configure other programs here.
};

... and keep the whole thing in a git repository to track changes and have it available for other machines.