It looks like this package should be in slot 2.0, not 1.9. Otherwise unmasking e.g. firefox4 and updating system doesn't work as there are existing applications depending on 1.9.
This is on purpose since xulrunner-1.9 and xulrunner-2.0 are not parallel installable. In addition, Firefox-4 has been end-of-lifed, and will be removed soon due to security bugs. Please use Firefox 5 instead.