Fiddler is not capturing web request from Firefox

You can also get the FiddlerHook plug in for Firefox that lets you handle how you want traffic to be routed through Fiddler

For recent versions of Fiddler, installing FiddlerHook manually is no longer necessary.

However, you still need to set up the proxy and certificate for Fiddler in Firefox. This blog post explains those steps.

https://docs.telerik.com/fiddler/knowledgebase/fiddlerhook

Configuring Firefox for Fiddler

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:

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: