I think the problem and solutions is here:
In short, you need to open caja as a superuser with sudo caja in a terminal
Navigate to "/usr/lib/firefox/" and delete "ubuntumate.cfg"
Navigate to "/usr/lib/firefox/defaults/pref/" and delete "all-ubuntumate.js"
Then, presumably, log out and back in
Check out the link above, though, to be certain for yourself
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