There is no setting for that any longer. Look for an add-on like this. https://addons.mozilla.org/en-US/firefox/addon/new-tab-homepage/

Took me 45 minutes of reading old posts just to find out that it had been changed as well as wiping out most of my extensions, my theme for Firefox, not allowing my password manager to install and a host of other crappy things. And along with this I have to see IE configuration of the NEWTAB with all those stupid blocks representing webpages. I really HATE that. Yup version 49.0.1 has really frigged with my 'personalized browser'. Why personalize it if all the geeks are going to do is screw you in the end? Off Topic? No, I wouldn't be hunting for this solution if we had been warned ahead of time that the geeks were going to screw with our 'personalized settings'.