it's PHP script which limit the downloads.
looks like the PHP script use the session cookie to archive this goal.
in firefox, open a tab, can access a URL to download a 50M file,
when open 2nd tab, access same URL, it'll wait until 50M file download completed.
if open the URL in another browser, it starts downloading without wait.
then back to firefox, open firebug+firecookie, delete all cookies, then can open 2nd tab to download same file at the same time.
so it's not relating to web server, but PHP script do it purposely.