Hello,
you are using 16.10, right?
Close Firefox.
Then go to /usr/lib/firefox/
and delete the file ubuntumate.cfg
(with sudo rights).
Now you can change your prefered homepage.
Does it work?
You can see the full history here.
Best regards
EDIT
Deleting the file with sudo rights:
- Open a terminal (Press ALT + F2 and type in terminal )
- In terminal:
gksudo caja /usr/lib/firefox/
(password needed) - Delete the file
ubuntumate.cfg
- Close the caja window.