Thank you for the review and for the suggestions!

The extension has been updated:
- The overlay now only shows on YouTube and not YouTube Music
- The settings now save correctly
- The opacity slider is now fully unlocked and can range from 0 - 100

Due to a quirk in how the extension was updated, you might need to allow it permission to youtube.com again (you can change the permissions in the browser's extension settings or by reinstalling the extension - Firefox will then prompt for permission).

Thank you again for the review and for bringing up these issues :)