Settings => Home Page => New windows and tabs => Firefox Startup (Default) Upgrade

[F1r3f0x_U23r]

Shortcuts (Sites you've saved or visited) in Firefox Start (Default) UI should have 10 columns instead of 8 columns for 16:9 screen sized monitors.

[Agentvirtuel]

Hello

Maybe ? with the help of https://www.reddit.com/r/FirefoxCSS/comments/v3bf11/increase_topsites_columns_in_new_tabpage userContent.css
And, about, browser.newtabpage.activity-stream.topSitesRow, take a look at https://connect.mozilla.org/t5/discussions/bookmarks-on-the-home-page/m-p/56607

You can paste userContent code into this lightweight editor
https://www.userchrome.org/download-userchrome-css.html
And Generate CSS file userContent.css

https://www.userchrome.org