At first restore the permissions of Firefox's folders:
sudo chown -R $USER:$USER ~/.mozilla/firefox ~/.cache/mozilla/firefox
Then try again to set it from Firefox's Preferences: Home → New Windows and Tabs:
- Homepage and new windows → Blank page (or your personal choose)
- New tabs → Blank page (or your personal choose)
And only if manual settings does not survive after reboot, then remove corresponding files from terminal with
sudo rm /usr/lib/firefox/ubuntumate.cfg
sudo rm /usr/lib/firefox/defaults/pref/all-ubuntumate.js
You can always bring this behaviour back with
sudo apt-get install --reinstall ubuntu-mate-default-settings
Reference link: Ubuntu MATE always forces search start page in Firefox