As said in the german post, and google/mozilla explains even more
Actually, it hasn't *really been needed* since firefox 4, ubuntu got rid of it in 2011 (Fri May 20), back then a interesting page to watch was another ubuntu thingie called - desktop-o-mozilla-rapid-release-maintenance blueprint just for changes etc.
I run firefox 36 (nightly) and can't remember the last time it was installed or needed
piper@x1:~$ apt-cache policy xulrunner*
xulrunner-12.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-17.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-8.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-21.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-26.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-11.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-16.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-2.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-7.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-20.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-25.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-1.9.1:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-1.9.2:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-10.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-15.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-1.9:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-6.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-dev:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-24.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-19.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-5.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-23.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-27:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-28:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-29:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-30:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-13.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-18.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-9.0:
Installed: (none)
Candidate: (none)
Version table:
xulrunner-22.0:
Installed: (none)
Candidate: (none)
Version table: