The release schedule of XULRunner closely matches that of Firefox. The planned features and their ship dates are listed below. These plans are subject to change based on the availability of additional hackers, etc.
This information is outdated
The XULRunner 1.8.0.x series
The XULRunner 126.96.36.199 and 188.8.131.52 releases were stable developer previews. They were based on the same gecko branch code as the Firefox 1.5.0.x series. They are no longer being actively produced or maintained, though building XULRunner from the MOZILLA_1_8_0_BRANCH should continue to work.
The XULRunner 1.8.1 series
The Mozilla Foundation/Corporation are not planning official releases of XULRunner from the Mozilla 1.8.1 codebase (used to make Firefox 2). However, the Eclipse AJAX Tools Framework is using this codebase to embed Gecko into Eclipse, and volunteers from that community have produced contributed builds of XULRunner.
XULRunner continues to improve on the Mozilla 1.9 codebase. Firefox 3 is built on top of a "private" XULRunner, i.e. XULRunner is installed in the Firefox application directory. The Firefox 3 executable has a -app switch to load a different XULRunner application.