It has nothing to do with the browser* - it is up to the server to handle.
Some servers, if you type domain.com will redirect you to
www.domain.com (e.g.
http://google.com adds the www )
Others, if you type
www.domain.com will redirect you to domain.com (e.g.
http://www.stackoverflow.com removes the www )
You may sometimes come across badly configured servers which do fail if they receive the non-www version, but I can't recall any examples of that (it did happen once to me recently, on some site that should have known better, but I forget the specifics.)
*Of course, because it is a convention, some browsers may well sometimes do stuff that involves adding www, but almost certainly what you're referring to is not browser-driven, but the server sending the redirect request. (You can use Firebug's Net panel to see this occur.)