Welcome, to Ubuntu MATE community, @Someone !
You can override such behavior by removing two files using terminal commands:
sudo rm -v /usr/lib/firefox/ubuntumate.cfg /usr/lib/firefox/defaults/pref/all-ubuntumate.js
and then restart Firefox.
Please note, that we do not pin/lock the ubuntu-mate-default-settings corresponding package version, so you should apply the above fix again when Ubuntu MATE 22.04 LTS will be released.