Mozilla introduced New Tab Page in Firefox 13 which shows thumbnails of most visited sites, these thumbnails are populated gradually once you start to surf the web with NTP turned on, there are concerns about NTP exposing private data in secure session Mozilla acknowledged that and promised a patch but still not released yet, don’t know why? O.K forget about that if you like Firefox New Tab Page which shows 3*3 (3 columns and 3 rows ) total 9 thumbnails by default, if you want to increase those thumbnails to access your most visited sites quickly, you can achieve that by installing New Tab Tools add-on.
Increasing Number of Rows of Columns for New Tab Page
Let me tell you AFAIK there is no hidden preference in about:config to increase those thumbnails, that’s why you need to install this extension if you want to increase them.
After installing the extension, open add-ons Manager then click on option of New Tab Tools extension which shows number of rows and columns allotted by Firefox, obviously these are 3 and 3.
Now here for Rows and Columns fields you can assign values you want New Tab Page to display of those thumbnails for e.g to for 4 rows and 5 columns design of NTP you need to enter 4 for Rows, and 5 for Columns now open NTP without need to refresh or restart of browser, your desired count of thumbnails presented on NTP.
Since I’ve cleared Firefox cache before installing this add-ons you can see most of thumbnails on above screenshot as empty but Firefox NTP in pic has 3 rows and 4 columns,.