Using Fiddler with Firefox

Once in a while I really should read documentation - the "hidden" features I might find on all the applications I use.

For some reason Fiddler stopped working with Internet Explorer, so I was actually reading the few doc and faqs that are on the Fiddler website (

If you don't know what Fiddler is, its an HTTP Debugging proxy. It logs all the traffic between your browser and the outside world. Its particularly useful when you're doing Flash remoting or remoting with AJAX.

Anyway... looking at the FAQs I noticed that it is possible to use Fiddler with any browser!! I decided to try it out with Firefox and it was really really easy to do.

  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. For me that would be : C:\Documents and Settings\Stephen\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 my full path is : C:\Documents and Settings\Stephen\My Documents\Fiddler\Scripts\BrowserPAC.js
  5. Hit the Reload button, then OK, close the options window and you are set!
You will need Fiddler to be open in order to access the internet now, but its easy to switch between your normal settings and automatic proxy when you don't need Fiddler's assistance.

I use firebug - it has so much built-in functionality.

" FireBug lets you explore the far corners of the DOM by keyboard or mouse. All of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including an error console, command line, and a variety of fun inspectors. "
# Posted By Nick Tong | 3/13/08 9:39 PM
Hey Nick,

Yeah I use Firebug too, but I'm finding that it has been doing some irritating things recently. I seem to have problems with the XHttpResponse not always showing up. Don't know why... It was fine when I was down in Windsor, but doesn't seem to work any more. Also I'm finding that whenever I open an new tab or window the firebug console is always open even though I've tried to tell it not to!
# Posted By Stephen Moretti | 3/13/08 9:39 PM
Thanks for the mention!

The main point of using autoconfig rather than hardcoding is that Fiddler rewrites the autoconfig file when it starts or shuts down. So you just need to restart Firefox to get it to update whether or not it tries to use Fiddler.
# Posted By Eric Lawrence | 3/13/08 9:39 PM
I searched in Google, seems fiddler didn't have linux version, ..
it's a pity.
# Posted By sods | 3/13/08 9:39 PM
Long live firebug :-D
# Posted By Alpha | 3/13/08 9:39 PM
Or you can just set an HTTP proxy to on port 8888
# Posted By Felix | 3/13/08 9:39 PM
Extremely helpful tip. I didn't like using Fiddler with IE. So, was looking for a tool to use with Firefox. And, delighted to find how to use Fiddler2 itself with Firefox.
Thanks a lot.
# Posted By Jack | 3/13/08 9:39 PM
I followed the steps,but finally,firefox can not connect to internet.How come....
# Posted By Elsa | 6/4/08 4:04 AM

this worked ok for me for a week...
and today it stopped - i mean that now fiddler doesn't catch requests from firefox :(
i still have same settings using fiddler as a proxy..
# Posted By stan | 10/3/08 2:17 AM
oh, sorry for spamming, but i found that the problem was in that javascript file...
when i used manual proxy ( - it again works fine :)
# Posted By stan | 10/3/08 2:21 AM
lol - no problem - glad you worked out what the problem was.
# Posted By Stephen Moretti | 10/3/08 8:40 AM
@elsa make sure you have fiddler running.
Once you've set up the proxy connection you will only be able to connect to the internet through fiddler.
Just reset your network connection if you don't need fiddler.
# Posted By Stephen Moretti | 10/3/08 9:20 AM
I found this one could be the best way for fx.
# Posted By MM81520 | 1/14/09 11:04 AM
Thanks for the tip. Works great.

The problem with Firebug (particularly with Flash I find) is that it just doesn't log the POST details in the request (somtimes) and you have to replay the call to see the result (which doesn't work if it didn't log the request!). Firebug has other great features but debugging requests isn't one of them in my experience.

Fiddler on the other hand just shows you exactly what happened.
# Posted By Andy T | 1/23/09 12:36 PM