Bugs are tracked in Mozilla Bugzilla.
Getting started with bug tracking in Launchpad.Ubuntu also tracks bugs for packages derived from this project: mozilla-firefox-locale-all in Ubuntu, xulrunner in Ubuntu, xulrunner-1.9 in Ubuntu, xulrunner-1.9.1 in Ubuntu, xulrunner-1.9.2 in Ubuntu, xulrunner-2.0 in Ubuntu.
