Hi There,
Just to chip in on this, since I began using Ubuntu Mate with 16.04, having installed from scratch, I never encountered this problem.
Since I performed a distro update/grade to 16.10 the other day, I too can count myself amongst the ranks who have found myself somewhat stuck with the Ubuntu MATE/Google homepage.
I myself wouldn’t see this as a problem per se where it not for the fact that I have to provide both Cloudflare and Google access via NoScript, which I’m ill inclined to do…
As a consequence I figured out a fix for this… Go to usr/lib/firefox and within that directory open (as Root/SU) ‘ubuntumate.cfg’.
Change the default URL to your own personal preference and save. Done!
Goes without saying you should probably do this whilst Firefox is closed 
Hope this helps…
Stu