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-connecti…
Increasing connection count will make HTTP 1.1 websites faster, but they'll also cause random rate limiting errors if you increase the value too much. How high you should set this value is up to you, but it's one of the first things you should reset when websites start acting weird in Firefox but not in other browsers.