Ocean10000 - 2:05 pm on May 9, 2012 (gmt 0)
"text/html; charset=utf-8" is the correct form to use.
IE doesn't couldn't find a match on how to render "text/html, charset=utf-8" so it asks for you to download it.
I am assuming Firefox just ignores what it doesn't understand, or has a code path that handles this case.
When you added the content type you more then likely added it with a "," instead of ";"