WordPress.org

Is this an official FeedBurner plugin?

No, it is not. I have written this plugin for a customer of my company ABA Web Design. It was listed here, because I find it useful and I will be very happy if the FeedBurner users like it too.

Does FeedBurner Widget support the new feeds2.feedburner.com URLs?

Yes, it does. The widget was updated to use the new URIs after the Google changes.

My FeedBurner account is configured to work with my own domain, using the MyBrand service? Is it possible to use the widget with MyBrand instead of the FeedBurner URL?

Yes, it is. FeedBurner Widget has now support for MyBrand. So, you can configure it very easy and use it with your own domain URL. (Thanks to mohanjith for the idea.)