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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