Its decent enough, but it WILL NOT SAVE the number of videos in the main youtube screen. I set it to five, and it always defaults to 6, no matter how many times i reinstall the addon, or restart firefox. So just be aware it does have its issues.
Some people noted that using the Refresh Firefox feature solved this issue. I have no idea why it happens as the extension's settings are simply stored in local storage but I'll see about moving it to extension API instead.
