platforms = platforms.linux; are specified in the
default.nix of Thunderbird. Since Thunderbird can also run on Mac, is there any reason why other platforms are being opt-out?
Did you confirm you can build it with Nix and run it on mac? (NixOS - Nixpkgs 21.05 manual)
If it does, in my experience it’s usually enough to send a PR loosening up the platforms to unix or all (I’m not sure if there’s an official policy here regarding which is preferred without specific knowledge of what it’ll run on). A package sometimes get set to a single platform because whoever wrote it couldn’t test it on any others.
If there are additional hurdles to building it on Mac, it’ll be even more common for someone without access to a Mac to just get the Linux version running, and leave it up to someone with a Mac to debug the remaining issues and PR a fix to get it working there.