If using flakes you could just for instance add another input. You can also set the input URLs to specific states of the nixpkgs repository by eg referencing specific commits. Then, you should be able to just, e.g., pick Firefox from unstable, another package from the current stable channel, and maybe a broken package from a pull request fixing said package.
If you are not using flakes you can also add system wide channels. IIRC you can then import these channels into your configuration.nix and select packages from the corresponding channels. But here the channels/inputs are not part of configuration itself in contrast to when using flakes.
There’s no command to just update all packages without changing the nixos version?
I’m a bit confused about what you actually want? Do you just want to update your packages, but stay on the same NixOS version? Just continue like before. Do you want to stay on your current version, but use some packages from the next version? That should also be possible if you somehow include that channel in your configuration.nix (though I don’t know how this would work in practice).
Personally, I just run with unstable though, then the releases aren’t that important.
I think I thought unstable would mean, well, unstable. Like nightly releases or something. Would you use unstable for Firefox?
Is that the equivalent to apt update and apt upgrade? I don’t want to apt dist-upgrade lol
