Open about:config in a new tab and make the following changes:

network.http.pipelining -> True
network.http.pipelining.maxrequests -> 10

Anywhere on the screen, right-click and add a new integer:

nglayout.initialpaint.delay -> 0

If you're using a proxy, also change the proxy versions:

network.http.pipelining.proxy.pipelining -> True

WARNING: Be conservative when setting your pipelining settings. To the untrained eye this feature looks like a DOS attempt on the server side and might get you blocked.