Fiddler can also be used with Firefox.  To configure Firefox to use Fiddler, follow these steps:

1. Install and Run Fiddler once. This will create some files, one of which you'll need.
2.
In "My Documents" you will now see that there is folder for Fiddler.
Inside there you'll find a Scripts folder. Grab a copy of the full path
to this Scripts folder. This would typically be : C:\Documents and
Settings\{login_id}\My Documents\Fiddler\Scripts .
3. Open up Firefox, go to Options under the Tools menu and then click on the Connection Setting button.
4.
Down the bottom of the connection settings is "Automatic proxy
configuration URL:" Select this option and in the box put the file path
to the Scripts folder followed by BrowserPAC.js which is a file in the
Scripts folder. So the full path is : C:\Documents and
Settings\{login_id}\My Documents\Fiddler\Scripts\BrowserPAC.js
5. Hit the Reload button, then OK, close the options window and you are set.
6.
To Run Fiddler and Firefox, select Fiddler from the Start|All
Programs|Fiddler menu pick list to start it and then start Firefox as
you normally would.