There is no current release of this source package in Debian "unstable". You can still report bugs, make translations, and so on, but they might not be used until the package is published.
XULRunner is a single "gecko runtime" that can be used to bootstrap multiple XUL + XPCOM applications that are as rich as Firefox and Thunderbird.
There are no registered releases for the XULRunner ⇒ 1.9.1.