Short: Make sure your DNS Server in System Preferences > Network is correct!

Firefox has been known to take a little while open new windows. FF2 improved it. And FF3 is just about instant now.

But then... all of a sudden... my FF (3b5) was taking a hideously long time to open a new window. I'm talking like 10 seconds. So I went back to 3b4. Same thing. How weird... because browsing still worked perfectly.

That week, we had some network changes. For a while we were using a Business Cable modem and a T1. I was using external DNS (behind the T1) so I was sure to go out the Cable and back in the T1 to test websites "from the outside".
Finally, we got rid of the T1. And since I was using the Cable modem for access, I never noticed a problem. Everything still worked. Except...
Firefox now took a very long time to open windows.

I didn't make the connection for over a week. But as soon as I updated my network preference with the correct DNS server, FF spring back to life.

I wonder what Firefox is doing when it opens a window. Any ideas?