[ChrisSD]

To reply to myself I've found that about:config has `browser.newtabpage.rows` which I can increase. When I do that and restart the browser it resets to 3 but `browser.newtabpage.activity-stream.topSitesCount` is increased and more sites are shown. Changing `browser.newtabpage.columns` doesn't appear to have an effect.

My question still stands but at least there's a partial workaround, even if it's potentially unstable.

EDIT This setting doesn't appear to be available on Android.

[aembleton]

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.

[ChrisSD]

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.