Manual installation

Chrome

1. Download the latest version or one of the releases
2. Extract the archive
3. Paste chrome://extensions/ into the address bar of your browser
4. Activate Developer mode
5. Click on the Load unpacked button and select the extracted folder

Firefox

1. Download the latest version or one of the releases
2. Extract the archive
3. Paste about:debugging#/runtime/this-firefox into the address bar of your browser
4. Click on the Load unpacked button and select manifest.json file from the extracted folder

Safari

https://github.com/code4charity/YouTube-Extension/issues/494#issuecomment-675098753