The "MaxConnectionsPerServer" Internet Explorer tweak has a Firefox equivalent that does not involve editing the Windows Registry. Firefox has a more reasonable value of 8 max connections per server (as opposed to 4 in IE). 8 connections is probably enough in most cases.

Still, if you want to change the number of max concurrent connections:

In the address bar, type: about:config
Look for: network.http.max-connections-per-server