Firefox 3 slow in ASP.NET Development Server

Ever since I started using Firefox 3 as my default “Browse With…” choice in VS2008, I’d noticed that pages loaded really, really slowly. I pulled out the big guns and fired up YSlow to figure out what the holdup was. We’re talking about 20-25 seconds to render around 200k of data.

YSlow showed me that each individual request to the server took over 1 second each, but when I loaded the same page in staging it took only 3ms. Clearly, the problem was related to Cassini, the VS2005/2008 built-in web server.

Sure enough, I found the answer after I figured out the correct question to ask (for the record, it was “Cassini Firefox Slow”). Since “Cassini” wasn’t the first thing I thought of when I began researching the problem, in the interest of increasing the visibility of the answer I’ll include it here, because it was driving me insane.

Enter “about:config” in the address bar to go to your Firefox configuration page and then filter by “ipv6″. Set “network.dns.disableIPv6″ to “true”.  Enjoy development again.