Support configuring other dirs as tmpfs, or a sync daemon about impermanence HOT 2 OPEN

    systemd.user = {
      # This is effectively what psd does
      services.firefox-persist = {
        Unit = {
          Description = "Firefox persistent storage sync";
        };

        Service = {
          CPUSchedulingPolicy = "idle";
          IOSchedulingClass = "idle";
          Environment = "PATH=${pkgs.coreutils}";
          # copy all files except those that are symlinks
          ExecStart = toString (pkgs.writeShellScript "firefox-backup" ''
            ${pkgs.rsync}/bin/rsync -avh --exclude={'user.js','chrome','extensions'} /home/doc/.mozilla/firefox/doc /persist/home/doc/.mozilla/firefox/
          '');
        };
      };

      timers.firefox-persist = {
        Unit = {
          Description = "Firefox persistent storage periodic sync";
        };

        Timer = {
          Unit = "firefox-persist.service";
          # Run every 15 minutes
          OnCalendar = "*:0/15";
          Persistent = true;
        };

        Install = { WantedBy = [ "timers.target" ]; };
      };
    };