Something I didn't find mentioning is increasing the number of parallel connections in Firefox. When I inspect the network and responses of FF, I often find connections were "blocked" (for several hundreds ms). Thus, increasing the number of persistent connections to a server will unblock those connections and allow the website to render much faster.
Go to about:config and change:
network.http.max-persistent-connections-per-server
from the default value of 6 to 24 or 32.
Experimental results for a website: (time until finish loading)
No of Conns Prime Cache w/o Cache
6 (default) 25 s 257 s
12 21 s 212 s
18 18 s 190 s
24 15 s 147 s