XULRunner was a great idea. It allowed us to create good-looking desktop applications with a native look&feel with an option to embed HTML and SVG content with only a simple server application.
At that time, it looked like a good idea to let the hard work of making a platform-independent UI browser on Mozilla and focus on the applications. Unfortunately, it wasn't.
Before it was killed, I created two technologies based on it. Phobos for Pharo and Squeak (see the screenshots): https://github.com/pavel-krivanek/phobos-framework
The second one was Seaside inspired XULJet for JavaScript: https://en.wikipedia.org/wiki/XULJet
At that time, it looked like a good idea to let the hard work of making a platform-independent UI browser on Mozilla and focus on the applications. Unfortunately, it wasn't.