So if this extension improves your web browsing, why aren’t these network settings improved by default in Firefox? This is because Firefox wants to adhere to the HTTP specification which says: “A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy.” Unfortunately, the specification (from 1999) is outdated at this point.
Second, on the server side it doesn’t matter. Even the most lightweight webserver can handle hundreds of simultaneous connections. On a high-traffic webserver, your visit is just noise to the server. On a low-traffic webserver, it’s only easier to handle your request: the server isn’t doing anything else anyway.
Microsoft also thinks two connections is not enough: Internet Explorer 8 supports six simultaneous connections (see IE8 for developers).
So, should your browser adhere to an old specification written in stone or should your browser comply with a changed world? You decide.
Kris Mainieri made a video about how to speed up Firefox with Tweak Network.
Link to extensión and more document : http://www.bitstorm.org/extensions/tweak/
PD: Thank to “minijedimaste” for you comment