In the meantime, you can open a Terminal and…
First backup the two files to your /home directory:
cp /usr/lib/firefox/ubuntumate.cfg /usr/lib/firefox/defaults/pref/all-ubuntumate.js ~/.
Then remove the source files:
sudo rm /usr/lib/firefox/ubuntumate.cfg /usr/lib/firefox/defaults/pref/all-ubuntumate.js
After restarting Firefox, you should now be able to set your desired home page.
Thanks to @lah7 for previously mentioning (in another thread) which two files to remove.