Comment 12 for bug 370093

For me removing the file (/var/lib/dpkg/alternatives/xulrunner) was what I finally needed to do to complete the install, so to summarize:

Delete the files listed here: http://packages.ubuntu.com/jaunty/i386/xulrunner-1.9/filelist
Delete /var/lib/dpkg/alternatives/xulrunner
resinstall manually by downloading xulrunner from: http://packages.ubuntu.com/jaunty/i386/xulrunner-1.9/download