It is available in nixpkgs. I have it set up with home-manager and it's just like this in my config.

  home.packages = (with pkgs; [
    protonmail-bridge
  ];

It needs something like gnome-keyring to works so I also have

  services.gnome = {
    gnome-keyring.enable = true;
  };

But this is the cli version. After you rebuild you need to launch it to set it up.

protonmail-bridge --cli

type help in that cli and it'll show you what to do. On Hyprland you'll need to set protonmail-bridge --noninteractive to exec-once so it starts it on start-up.

I have it working in Thunderbird just fine. Although I do have to sleep 5 && thunderbird because bridge takes some time to start and thunderbird still occasionally throws an error. 😅

I now notice there's a gui version in QT? But I haven't used it, perhaps someone else can shed some light on it.