I've changed the `browser.newtabpage.activity-stream.topSitesCount` to 36 which fills my screen with 6x6 boxes filled with my top sites. Except, for the last box which is always empty!
It seems that there is a hard limit in Firefox of 35 top sites.
Did you try restarting your browser? I've just tried setting rows to 12, which sets topSitesCount to 72. This worked for me after restarting the browser.
