Outside of Firefox, you could watch the traffic in a packet sniffer, if it's your personal workstation. The old workhorse tcpdump is a bit unwieldy, modern graphical tools like Wireshark may be more friendy at least for getting a feel for what you really want to look for; once you know that, you can go back and script tcpdump and create your own little custom application on top of it to show what's pending and how long things are taking. (If you go down this route, I'd really like to try it out!)