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.
404 for the full changelog :/