Hi @maximuscore,
I am using UM 16.04 with Mate 1.14 and do not have your problem so I can only assume that the bug isn’t having an effect on everyone!!. 
This is the same on both my tower PC with an Nvidia GPU and on my mini notebook with an Intel GPU!. 
I am on 16.04 64 bit with full upgrades and updates, without this problem
Seems to be just a 16.10 bug, ubuntu-mate-default-settings for 16.04 (v16.04.5.3) doesn’t even contain the configuration files:
/usr/lib/firefox/ubuntumate.cfg
/usr/lib/firefox/defaults/pref/all-ubuntumate.js
They’re responsible. It’s safer to delete them with sudo then uninstall ubuntu-mate-default-settings which contains other defaults like the panels in MATE Tweak.
It would have been helpful @SkipF if you specified which Ubuntu MATE version in your opening post. 
Regardless of version, I did experience this bug too on 16.04. It is unrelated to MATE versions but rather this is not the correct approach to set defaults as a one off.
I’ve only had this problem since installing 16.10 on my laptop. I could live with it until it was fixed, except that now all the other devices I’ve connected with Firefox Sync – including an Android tablet and a Win10 desktop – have had the home page changed.
However, while reading this thread, the thought occurred to me that if /usr/lib/firefox/ubuntumate.cfg was causing the trouble, and deleting it didn’t help, how about editing it? Couldn’t make matters any worse…
Voila! Six hours later (rather than the usual five minutes) and it hasn’t changed and it’s right on the other devices. Hopefully, future updates won’t restore the bad behavior, but at least it’s an easy fix.
It doesn’t affect me. I’m on 16.04. So, definitely post-16.04
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
Is there any chance of this bug being fixed? It’s certainly a nuisance, especially for non-expert users. (I can fix this, my parents rather less…)
It’s kinda overdue for a fix, considering it was reported very early in the 16.10 development cycle.
I never had the problem on 16.04 just 16.10.I deleted the 2 files lah7 suggested and the problem was solved.
Agreed, lah7’s suggestion fixed it for me too.
Hi
change the input in
/usr/lib/firefox/ubuntumate.cfg
Thank You Eu_Lo! That was really annoying bug.
Unfortunately, still triaged. 
I’ve had a few attempts myself trying to fix this, but still no luck. 
Any Firefox configuration experts out there or anyone with Mozilla contacts?
It should be a simple fix for a simple problem, but this documentation is baffling me. What should happen is that we set a “default” (provided by the ubuntu-mate-default-settings package) for new Firefox profiles, but it seems that the configuration at the moment just overwrites regardless. 
I’m at a loss with this, but if anyone fancies having a crack at fixing this heated bug, please go ahead! 
These are the culprits:
/usr/lib/firefox/browser/defaults/preferences/all-ubuntumate.js/usr/lib/firefox/ubuntumate.cfgDoes this guide help? https://mike.kaply.com/2012/08/29/setting-the-default-firefox-homepage-with-autoconfig/
Just a heads up, Ubuntu GNOME suffered from the bug, fixed it and @Wimpy applied the fix for Ubuntu MATE.
I’ll be testing this out later to be absolutely sure the fix works. I’m not convinced as my attempts couldn’t get a brand new profile to use the correct default. 
Looks like you can configure the default home screen in Ubuntu MATE once again. However on clean installs our preferred home page is not applied. Testing the same solution on Windows works. So it would appear there is a bug with defaultPref on Linux in Firefox.
in about:config, search for home, in browser.startup.homepage paste your preferred homepage url into ‘value’
So far, changing /usr/lib/firefox/ubuntumate.cfg has worked with a fresh 16.10 installation. Changing about:config browser.startup.homepageshould work, but it’s overwritten, at startup, at least under my 16.10 install. YMMV