I hate firefox newtabs, because anyone can see what kind of stuff i was watching before.
To turn off firefox newtab miniatures, type in your browser url: about:config , then in search type: newtab and change browser.newtabpage.enabled to false and browser.newtab.url to about:blank . Restart Firefox and profit! When you open new tab you will see blank tab. See Picture 1
|Picture 1 - Firefox configuration|