Newbie: Download Manager API's?

[Chris Giblin]

Hi,

I want to develop a custom download manager - can anyone point to the
set of API's I would need to use?

I'm no stranger to the HTTP protocol, MIME, etc., but this is the first
time I will trying to develop an extension (plugin?) to a browser. I
have surfed around, but have not gotten a grasp on what API's I shoul
dbe looking at.

Thanks, c