Mozilla Firefox proxy settings are configured automatically using the Charles Autoconfiguration Add-on. The add-on is available for download from the Charles Download page.

The Charles Autoconfiguration Add-on adds a Charles option in the Tools menu in Firefox; you can view the current status in that menu. However you will not usually need to use that menu as the add-on will silently take care of configuring and unconfiguring Firefox.

When you first install Charles, it will check for and prompt you to install the Firefox add-on. If you don’t install the Firefox add-on immediately you will later need to enable Firefox configuration in the Proxy Settings in the Proxy menu.