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.