The solution was to reboot our firewall. I don't know why this affected some browsers but not others.
I've noticed that the CCSF home page loads very slowly. I reported it to Tim Ryan, our technical manager, and got the reply "What browser are you using?".
I thought that was a silly question, but I decided to test it anyway. I usually use Chrome, but I tested the page in Safari and found the same results.
When I told Tim that, he said Firefox would work better. Again, that sounded ridiculous to me, but I downloaded and installed Firefox to test it.
And it's true! You can probably see it yourself right now, until it's fixed.
Click this link in Chrome or Safara. It will take 80 seconds to load the page:
Now click that same link in Firefox. The page loads in 1 or 2 seconds.
I used Wireshark to capture the load in Chrome and saw this:
Packet 150 does a GET /Images/facebook.jpg, and after that there is a 45-second pause.
To see why, I followed the TCP stream of packet 150:
This image redirects to itself endlessly--a redirect loop. And apparently different browsers handle that problem differently. Chrome and Safari hold up the page struggling with that problem, but Firefox loads the rest of the page quickly.
It loads over HTTPS, so Wireshark doesn't let me see the HTTP requests, but Chrome Developer Tools shows that the page loads partially, then is blocked for 1.3 minutes, at which time some more elements load, then it's bloked until 2.6 seconds when the rest of the elements load. This page does not include the facebook.jpg image, so there must be other resources on the CCSF website that cannot load.