I came across a Reddit Thread recently which included this gem:
From the comments that have been posted on this thread and what I found on the Mozilla forums so far:
1- In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.
2- Set browser.newtab.url to about:blank>
3- To disable the callbacks to tiles.cdn.mozilla.com without enabling the "do not track" feature you need to remove the address from browser.newtabpage.directory.ping and browser.newtabpage.directory.source
So i've now done both of the above and feel much better.
The Reddit page linked to a ZDNet.com article talking about Mozilla's new Advertising strategy. I for one don't need a third party tracking what I do when I click on 'new tab' ... ! Interestingly there's also a move to remove browser.newtab.url due to "Abuse" which seems to be in itself, contentious, but it's possible you'll have to use an addon to achieve the above, at least in part, in the near future.