This happened with a recent Nightly update. I am on Firefox Nightly 60.0a1 (2018-02-11) (64-bit). Before a update, one or two days ago, I had 12 pinned sites (6 on each row, 2 rows, for a total of 12) for my new tab page. Now with the latest Nightly update, I have my 12 previous pinned sites, but 4 random sites are added, for a total of 16 - which looks hideous. Refer to the image for more details. I can't change the no. of tiles in the activity stream, since the flag for that is gone. How do I resolve this? This completely ruined my browsing experience.