XULRunner is a runtime package that can be used to run XUL+XPCOM based applications. Automated builds of it have been produced alongside Firefox since 2006, but it has not been a supported or resourced product for many years. We've continued to produce automated builds of it because its build process also happens to build the Gecko SDK, which we do support and maintain. This will change soon, and we'll start building the Gecko SDK from Firefox instead (bug 672509). This work will land on mozilla-central during the 42.0 cycle, which means that when the 41.0 cycle ends (September 22, 2015), automated builds of XULRunner will cease.

If you are a consumer of the Gecko SDK this means very little to you -- we will continue to produce it with every Firefox release.

If you are a consumer of the XULRunner stub this means that you will no longer have a Mozilla produced version after 41.0. For folks in this group, you have two options:

• Change your app to run through the stub provided by Firefox. Many apps will continue to work as before by simply replacing "xulrunner.exe application" with "firefox -app application.ini".
• Build XULRunner yourself.