A: 

Have you tried sending the response header and the file in the same send command? Is there a way you can tell what firefox is receiving. You could try writing, using some HTTPRequester class, a console app that sends a HTTP request to your server and see what its getting in return.

bobber205
I could try that and I think it would probably work, but is it a requirement to do it that way? What if the file is too big to completely buffer into memory and you have to do multiple sends?
Silmaril89