This extension contains a collection of XUL demos that used to be available as a web page, but can no longer be viewed in browsers since support for Remote XUL was disabled.
The content of this extension is based on the work of Alice Corbin. The original repository can be found here.
Simply download the contents of the "src" folder and pack the contents into a .zip file. Then, rename the file to .xpi and drag into the browser.
Check the releases page of this repository.