The "Ad-block" extension to Firefox, properly used, gets rid of these ads, use it in addition to the customized FT GreaseMonkey extension (which an expert could presumably modify to take care of these ads as well).

I agree with those previous posters who noted that the ads are likely being made more and more obnoxious to pressure people to buy the "ad-free" version of FT when it appears; that is an obvious consequence of the decision to offer a paid version.