Use defaultPref() for Firefox defaults

This technically fixes the problems raised here:
https://bugs.launchpad.net/ubuntu-mate/+bug/1605887
https://bugs.launchpad.net/ubuntu/+source/ubuntu-mate-settings/+bug/1721965

However, the actual defaultPref() functionality is broken due to this upstream bug report:
https://bugzilla.mozilla.org/show_bug.cgi?id=947838

usr/lib/firefox/ubuntumate.cfg

@@ -1,2 +1,2 @@
 //
-pref("browser.startup.homepage", "https://start.ubuntu-mate.org");
+defaultPref("browser.startup.homepage", "https://start.ubuntu-mate.org");