Mozilla Firefox is unlike most Windows browsers in that it does not use the system’s proxy and certificate settings by default. That means that Firefox may require some additional configuration to work properly with Fiddler.

To get Firefox running with Fiddler, you need to:

• Configure Firefox to proxy its traffic to Fiddler.
• Configure Firefox to trust Fiddler’s root certificate.

Fortunately, both of these are simple tasks.

Configuring the Proxy

Firefox’s proxy settings are found by opening its Tools menu, clicking the Options item, and opening the Advanced settings. Select the Network tab. At the top of the tab, click the Settings… button to the right of Configure how Firefox connects to the Internet.

At this point, you have two choices:

• You can configure Firefox to respect the system proxy settings.
• You can manually configure Firefox to point at Fiddler.

If you’d like Firefox to work like all other browsers, sending traffic to Fiddler when Fiddler is set to Capture Traffic, simply choose the Use system proxy settings option, then click OK. This is the recommended configuration:

Alternatively, you may manually configure Firefox to send traffic to Fiddler by entering the IP address and port (by default, 127.0.0.1 and 8888) in the appropriate boxes:

If you configure Firefox to use this Manual Proxy configuration, you’ll have to come back and revert these settings after you close Fiddler. If you forget to do so, Firefox will not load pages and instead show the following error message:

Trusting the Certificate

If Fiddler is configured to decrypt HTTPS traffic (Tools > Fiddler Options > HTTPS), you will find that attempting to load a HTTPS page in Firefox yields a scary warning message:

This message is shown because Firefox does not use the Windows Trusted Certificate Authority list; it instead has its own list of trusted certificates.

To avoid this warning page, we can reconfigure Firefox to trust the Fiddler root certificate.

First, we need a copy of Fiddler’s Root certificate. Inside Fiddler, click Tools > Fiddler Options. On the HTTPS tab, click the Export Root certificate to Desktop button at the bottom. This will create a file named FiddlerRoot.cer on your desktop.

Next, we must import this certificate into Firefox’s Certificate Manager. Firefox’s certificate settings are found by opening Firefox’s Tools menu, clicking the Options item, and opening the Advanced settings. Select the Encryption tab. Click the View Certificates button.

In the Certificate Manager, click the Authorities tab, and the click the Import button at the bottom:

In the Select File containing CA certificate(s) to import box, choose the FiddlerRoot.cer file that you saved to your desktop a moment ago, then click Open.

In the screen that appears, check the Trust this CA to identify websites box at the top, then click OK.

With this change in place, Firefox will now trust the Fiddler root certificate. Visiting HTTPS pages in Firefox with Fiddler capturing will no longer show certificate error pages.

What about FiddlerHook?

You may have noticed that installing Fiddler also installs a Firefox extension named FiddlerHook. This extension is meant to help simplify the configuration of Firefox to use Fiddler. However, over the years, Firefox has started disabling extensions by default, and you may find that your FiddlerHook extension isn’t enabled. If it is, no worries—the last two sections have shown you how to manually configure Firefox.

If you’d like to use FiddlerHook instead, enable it using Firefox’s Tools > Add-ons > Extensions screen, restart Firefox, and read on.

When enabled, FiddlerHook adds a Monitor with Fiddler command to Firefox’s Tools menu.

The menu offers the ability to change whether Fiddler is used as Firefox’s proxy, and permits you to launch Fiddler directly. If the Show StatusBar item in the Monitor with Fiddler menu is ticked, and Firefox’s View > Toolbars > Add-on Bar option is also ticked, Firefox’s status bar will show the current state of FiddlerHook at the bottom right:

This feature works by periodically checking whether Fiddler is running, and automatically adjusting the proxy settings based on that check. If you click on the status bar panel, a popup menu enables you to change your FiddlerHook settings. It also provides one-click entry points to clear the Firefox cache and cookies:

FiddlerHook also enables you to add a button to Firefox’s toolbar that launches Fiddler. To show this button, right-click the Firefox toolbar and choose Customize. Drag the Fiddler button to the desired location:

In the very latest version of Fiddler (v2.4.3.7+), FiddlerHook also simplifies the process of trusting the Fiddler root certificate. Simply export Fiddler’s root certificate to the desktop as described above. Then, inside Firefox, click Tools > Monitor with Fiddler > Trust FiddlerRoot certificate.

This will automatically import the Fiddler root certificate. Simply check the Trust this CA to identify websites box when prompted, then click OK.

-Eric Lawrence