Improve Darwin story for firefox and thunderbird

[booxter]

Description

1. Allow to set null as package for thunderbird. (Common on Darwin when it's pulled from e.g. Homebrew. No need to build a dummy package.)
2. Use -unwrapped packages for Darwin where wrapped versions are not supported in nixpkgs.
3. While at it, fix example for thunderbird.package to list a package that actually exists (thunderbird-91 was removed a while ago).

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC