The latest version of Charles is 4.1.1. Read the Release Notes.
Paid Upgrade for Charles 3 to Charles 4
Charles 4 was a major update to Charles 3, and it is a paid upgrade. Please download a trial of Charles 4; we think you're going to love it! If you would like to continue using Charles 4, you will need to purchase a license upgrade.
You can continue to download the latest release of Charles 3 from the Previous Release page.
Mozilla Firefox add-on
Charles requires an add-on in Firefox to autoconfigure Firefox's proxy settings. Charles will automatically prompt you to install the add-on when you run Charles. If the add-on is not automatically installed, you can manually activate Mozilla Firefox Proxy in the Proxy menu. Alternatively you can download and install the add-on manually.