Firefox 27.0.1

I upgraded Firefox from 24.0 to 27.0.1 (a big while since last update...). Now I note something strange in the New Tab window. This window shows a "preview" of 9 of the most frequently visited websites according to Firefox's registry in the form of 9 small windows, right? Well, now the 9 previews never change, not even when restarting Firefox or rebooting OS; and I visit different pages each day. And for example, for websites that vary its front page frequently like Youtube, its preview on New Tab window doesn't change neither.

Have I done something wrong? How can I fix?
Thanks.