XULRunner is a runtime that can be used to bootstrap multiple XUL + XPCOM applications that are equal in capabilities to Firefox and Thunderbird.
Starting with version 3.0, Mozilla Firefox uses a "private" XULRunner,[5] meaning the framework is installed locally in the application directory.
The Google AdWords Editor used XULRunner[6] until 2010,[7] as did Evergreen ILS, an open-source library automation system.
[9] As of the beginning of 2016, it had been dropped from the package database of most Linux distributions, including Gentoo,[10] Debian,[11][12] and Ubuntu.
[16] Despite this deprecation, the latest versions of Firefox and Thunderbird can still run XULRunner applications with the -app command line argument.