There are concerns over Firefox New Tab Page exposing private data if you don’t want Firefox from capturing thumbnails you can prevent thumbnail capture of Firefox by create and enabling a hidden preference in abut:config. Mozilla introduced New Tab Page feature in Firefox 13 which shows thumbnails of most visited sites from user’s browser history, since this feature is not unique and offered by other browsers like Opera and Chrome, despite not getting positive response from users Mozilla getting criticism to this as thumbnails on NTP exposed private data, Mozilla seems to have fixed this in Firefox 14 by excluding capturing of thumbnails that connects to secured sites, you can disable Firefox New Tab Page by setting browser.newtab.URL preference to about:blank. Another issue raised by users with NTP is thumbnails folder in Firefox profile folder getting bigger and bigger day by and they some how want to stop Firefox from capturing thumbnails? is there a way to do this? yes continue to read below for more details.
How to Prevent/Stop Firefox from Capturing Thumbnails
All you need to do is visit about:config page in Firefox bar and create Boolean preference browser.pagethumbnails.capturing_disabled and set its value to true.
If you don’t got it, let me clear this for you right-click anywhere on about:config page browser for New>Boolean and enter above preference and click ok then set its value to True.
From now onwards no new thumbnails will be created and mind you still need remove the old thumbnail by clearing the browser history.
P.S. In fact we’ve not seen thumbnails folder in Firefox profile folder either.