There's another way of doing it, a bit more technical.

Open Firefox - type in the address bar:

about:config

Continue through the "here be dragons" page.

Search for "newtab"

You'll have two options there:

browser.newtab.url
browser.newtabpage.enabled

My options (to open a blank page when you open a new tab):
browser.newtab.url - about:blank
browser.newtabpage.enabled - false

Restart firefox after changing those and it will just open a blank page rather than the top sites page. You could also change the newtab url to http://www.google.co.uk to open Google each time you open a new tab

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