Hi  !

You can increase the number of rows and columns :

Type in the address bar about:config (press Enter) (promise to be careful, if asked)

Type in the search bar and look for the preference :

browser.newtabpage.columns

and set its value to a higher number

And you can do the same with :

browser.newtabpage.rows

In adddition to the above - you might find this interesting :

https://www.ghacks.net/2017/09/13/firefox-57-disable-highlights-on-the-new-tab-page/