There is no built-in way to change the number of page thumbnails on the tab. However, someone might develop an add-on to tweak the new tab page if there's a lot of interest.

where do we vote/sign up?

Hi DextroWombat, you might look at existing add-ons in the speed dial/fast dial category to see whether any of them are now compatible and offer more flexibility. If they don't increase the number of thumbnails, maybe those authors would consider adding that feature? They have a big head start on other add-on developers.

Future Firefox versions will have that feature.

See this bug report for the progress:

• bug 752841 - [New Tab Page] make the number of tabs adjustable

(please do not comment in bug reports)

You set the number of rows and columns that appear on the about:newtab page by modifying these prefs on the about:config page via a double-click in Firefox 17 and later, Firefox 17 ESR supports this.

• browser.newtabpage.columns (3)
• browser.newtabpage.rows (3)