With Firefox 13, Mozilla has changed the behaviour of new tabs as we had mentioned last month. Now when opening a new tab, a list of recently visited sites are displayed including thumbnails of said sites. There has been a lot of demand for that feature by Firefox users as Chrome has had it for long time and I think Opera does as well. To turn it off (permanently) and go back to a blank page follow these directions:
1) Type in “about:config” (without the quotes) in the browser address bar and then press enter.
2) Click on the warning “I’ll be careful, I promise” and on the next page, type “newtabpage” (without the quotes) in the search box.
3) Double click on browser.newtabpage.enabled preference to change to False.
Source: mozillaZine: Firefox Support