In the URL line, type "about:config" without the quotes. Scroll down to network.http.proxy.version. Change the value from 1.1 to 1.0.
While you are in the neighborhood, make these changes as well:
network.http.max-connections = 64
You can play with the network-http.max-connections-per-server number if you want to see if you can do better. I've had it up to 64 with network.http.max-connections as high as 256. The only problem is you might run into trouble with sites that limit your maximum connections. Example are the extension and theme download sites for mozilla. The numbers above should provide good performance without side effects.
I would not push persistent connections above 12. That can cause problems on lots of sites, particularly above 15. And I would leave the connections-per-proxy settings alone.
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