With New Tab Pro you can customize the number of the tiles on the New Tab page (about:newtab), right on that page via the buttons at the center of each side of the page. Or use the Options page of this extension to get the same effect.
Here is a little more information about this add-on for Firefox:
A limitation period of visits (unpinned thumbnails) can be defined via the couple of the extra buttons: a day ago or a past week. Also, you can clear the list of the blocked links by the Options page. This list comes from the “X” button in the top-right corner of the thumbnails of the New Tab page.
New Tab Pro is a great add-on to get just a little more tweaking and customizing out of your tabs. You can pick it up at the Firefox Add-ons website.