Go to Firefox settings, on the left hand panel click on home, then scroll down until you come to Firefox Home Content. You should see shortcuts and under that Sponsored Links. Untick the blue box next to that and you should be all set.
Another way of doing it is to set all the four lines to show on your home page. Pin every box of the four lines, then set the first lines with the sites you want. Then reset how many lines you want shown according to the sites you want shortcuts to. It worked for me.
