Based on the official release notes , version 3.3.9 and its nearby iterations focused on:
: Regular updates to ensure compatibility with major sites like YouTube and SoundCloud. Downie 3.3.9
Downie 3 is now a legacy version. If you are using a modern macOS, it is highly recommended to use for better performance and support for the latest website security updates. Charlie Monroe Software - Downie - Release Notes Based on the official release notes , version 3
: Support for extensions in Safari, Chrome, and Firefox for one-click downloading. 4K Support : High-resolution video downloading capability. How to Use Downie Based on the official release notes