As I mentioned yesterday, I recently upgraded to Firefox 3.0 and immediately found a few issues with the application we are testing.  After going through the standard debugging steps, I pulled out Fiddler, an awesome (and free) application to analyze the HTTP traffic.  Now, Fiddler is a fantastic tool but has a lot of quirks about when it decides to analyze traffic.  The most common issue is that it doesn't analyze traffic from your localhost.  The Fiddler site says this is because "IE7 and the .Net Framework are hardcoded not to send requests for Localhost through proxies".  Since most of you reading this (including myself) are developers using Cassini and testing on our localhost, fixes are in order.  Some of the basics include:

• Putting a period between the localhost and the colon (before the port number)
• Routing your traffic through an outside site like somesite.com
• Putting an entry in your host file that points at your localhost, but with a name (For example, 127.0.0.1  Cassini)

I've seen all of these work to some degree.  However, I had never realized that Fiddler didn't seem to pick up Firefox traffic AT ALL.  I looked into it a bit and found a quick older fix for this on the "Life of a Java Disciple" blog.  Updated for Firefox 3, the steps are as follows:

• Go to My Documents > Fiddler2 > Scripts.  There should be a file called BrowserPAC.js.  Make note of the full path for this file.
• Open Firefox 3.0.  Open the Tools menu and click Options. 
• Under the Network Tab, click Connection > Settings.
• Under Setting > Choose "Automatic proxy configuration URL" and put the full path to BrowserPAC.js in the text box. 
• Click OK

Open Fiddler and try Firefox again.  You should start seeing requests show up.  Hope this helps.

UPDATE: I removed the recommendation for "Using the name of your machine, rather than localhost" after I was assured by a co-worker that this doesn't work now (if it ever did).  This is why it's good to be surrounded by smart people.