Capturing from Mobile Firefox Browser
By default, the Android version of Firefox does not respect and trust the Fiddler Certificate Authority (CA). As a result, once the proxy configuration is in place, you will see a warning in Firefox for Android that prevents any further browsing due to security concerns. To resolve that issue and to be able to capture HTTPS traffic from Firefox for Android, you need to explicitly instruct the mobile browser to trust user-installed CAs through the following steps:
On your mobile device, open Firefox for Android and go to Settings.
Tap on About Firefox
Tap the Firefox logo seven times
Go up one level. You can now access the Secret Settings (near the bottom of the Settings menu).
Within the Secret Settings toggle on the Use third party CA certificates.
Restart Firefox for Android.
The described workflow works in Firefox for Android version 90.0 and above.
Now that Firefox for Android is configured to trust user-installed CA, you can capture its HTTPS traffic with Fiddler Everywhere.