Archlinux is switching towards XULRunner for applications that want to use the Gecko engine as embedded library. XULRunner is more API stable than firefox and should give us less problems in the future.
While upgrading your system, you might notice pacman wants to remove firefox and install xulrunner: this is a bug in pacman, upgrade to mozilla-firefox-18.104.22.168-2 first and it won't complain about removing firefox anymore.
Another change that has been made in the structure of mozilla applications is the move of plugins from /opt/mozilla-plugins to /opt/mozilla/lib/plugins. The reason for this is that we want to have all mozilla related stuff INSIDE /opt/mozilla, not somewhere else. Most plugins have been updated to reflect this change. Please report bugs when you find plugins that install in /opt/mozilla-plugins anyways.