Charles Proxy is a network traffic capture tool that can capture network requests and responses from your computer and mobile phones. When you use the proxy program on your computer, it is able to capture network traffic through all browsers, IE, Edge, Chrome, Safari, etc. And you don’t need to configure the proxy software or the web browsers. Charles will set proxy into our system. This is global proxy setting that affects all browsers on the machine.
Most third-party web browsers don’t include their own proxy settings. Instead, they follow the system proxy settings, sharing the options with Microsoft Edge, Internet Explorer and the Windows system. Firefox is exceptional. As the most popular web browser, Firefox provides users with flexible options they can tweak to suit their special needs. For example you can specify proxy setting for Firefox to override system proxy settings without affecting other web browsers or web apps on the same machine. As a result you may find Charles or other proxy programs can’t intercept this browser’s network traffic. When you run into this trouble, the fix for this is very easy. Just jump into its settings to force it use system proxy settings.
From the Menu bar of Firefox for Windows, click Tools > Options. The browser preferences opens in a new tab. Scroll all the way down to find Network Settings section. Click Settings, the Connection Settings dialog shows up. Here you can configure proxy access to the Internet through Firefox. Enable ‘Use system proxy settings‘ option or select ‘Manual proxy configuration‘, then set HTTP Proxy to 127.0.0.1, type 8888 in the Port box. Click OK button to save the change and close this Connection Settings dialog.