Not sure if anyone has come across this yet but this change is guaranteed to annoy some...and I have come across this article that may help those that really do not like ads in their browser sessions:
"Updated 14. November 2014 - 22:56 by v.laurie In an effort to develop new sources of advertising revenue, Mozilla has started displaying ads on the page that is used for opening a new tab in the Firefox browser. This new practice was started in Firefox 33.1 and is described in the Firefox message shown below."
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