Mozilla Firefox does not use the system’s proxy and certificate settings by default. It may require some additional configuration to work properly with Fiddler.

Configuring the proxy settings in Firefox:

1. Open Firefox, then select the Customize your new tab page icon.
2. Click on General, then scroll down to the Network Settings section and select Settings.
3. On the Connection Settings screen, choose the Use system proxy settings option, and then click OK.

Trusting the certificate:

1. Open Fiddler 4, then go to Tools > Options...
2. Go to the HTTPS tab.
3. Click on Actions, then select Export Root Certificate to Desktop.
4. Open Firefox again, then select the Customize your new tab page icon
5. Click on Privacy & Security, then scroll down to the Certificates section and select View Certificates.
6. On the Certificate Manager screen, click the Authorities tab and select Import...
7. Choose the FiddlerRoot .cer file saved on your Desktop, then select Open.
8. On the Edit CA Certificate trust settings window, check the This certificate can identify websites box and select OK.

With this change in place, Firefox will now trust the Fiddler root certificate and it will work properly with Fiddler.