It’s a bit hidden, but you can actually change this in the new version:
- type “about:config” in the address bar
- click the “I accept the risk” button
- In the search field at the top, type “browser.newtabpage.activity-stream.topSitesCount”
- double-click the item that appears in the list
- enter the number 18 and click “OK”.
You can actually have more than three rows if you want as well – entering 24 gives you 4 rows, 30 gives you 5 rows, etc.