Happy 43rd Mal!
This space western TV series only ran from 2002 to 2003 on Fox but it has been a cult favorite ever since it hit the small screen. Nathan played the main role of Malcolm Reynolds (Mal) the owner of the starship Serenity.
This space western film which was released in 2005 was directed by Joss Whedon and based on his TV creation, the aforementioned Firefly. Nathan of course reprised his role as the captain of the starship upon which the film get its name.
3. Dr. Horrible’s Sing-Along Blog
Created and directed by Firefly creator Joss Whedon, this miniseries was released in 2008 exclusively for the internet. It can now be found on Netflix (as least it was on there the last time I checked). Nathan played Captain Hammer, the archenemy of the main character, supervillain Dr. Horrible, who was played by Neil Patrick Harris.