Type about:config in the address bar.
Change these settings:
| network.http.max-connections | 32 |
| network.http.max-connections-per-server | 16 |
| network.http.max-persistent-connections-per-proxy | 16 |
| network.http.max-persistent-connections-per-server | 16 |
| network.http.pipelining | true |
| network.http.pipelining.maxrequests | 8 |
| network.http.proxy.pipelining | true |
UPDATE: This can now be done easily with the Tweak Network extension by choosing the Power profile.
UPDATE 2: Added the pipelining that I somehow left out and removed nglayout.initialpaint.delay