I think this is what you may be looking for:
// limit the absolute number of http connections.
pref("network.http.max-connections", 24);
// limit the absolute number of http connections that can be established per
// host. if a http proxy server is enabled, then the "server" is the proxy
// server. Otherwise, "server" is the http origin server.
pref("network.http.max-connections-per-server", 8);
// if network.http.keep-alive is true, and if NOT connecting via a proxy, then
// a new connection will only be attempted if the number of active persistent
// connections to the server is less then max-persistent-connections-per-server.
pref("network.http.max-persistent-connections-per-server", 2);
// if network.http.keep-alive is true, and if connecting via a proxy, then a
// new connection will only be attempted if the number of active persistent
// connections to the proxy is less then max-persistent-connections-per-proxy.
pref("network.http.max-persistent-connections-per-proxy", 4);
// amount of time (in seconds) to suspend pending requests, before spawning a
// new connection, once the limit on the number of persistent connections per
// host has been reached. however, a new connection will not be created if
// max-connections or max-connections-per-server has also been reached.
pref("network.http.request.max-start-delay", 10);
Took alot of looking at config files for Firefox to find this.
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