Where would an ad cookie or script hide?

I am an avid reader of the New York Times online. I look at it several times a day and read more than one article each time. My New York Times experience is being ruined by the ads placed by two companies. Dot & Bo sell knockoffs of famous modern furniture pieces. Mack Weldon sells underwear. For perhaps three years, one or the other of these ads has appeared on nearly every page of the Times I read. I have seen each ad thousands of times. I have never clicked on either of them. But the ads keep coming. Dot & Bo and Mack Weldon have crossed the line from marketing to harassment. So, my question. Suppose there is a cookie or script that was deposited somewhere in my computer, an iMac running OSX 10.7 Lion, triggering the appearance of these ads in the online pages of the TImes when I open it. Where would it be and how would I get rid of it? I realize that these ads are probably controlled on the content side of things, that a media placement company hired by the advertisers buys the space and the ads go up. But maybe there's something on my end I can do about this. The ads appear whether I browse in Safari or in Firefox. I would be so grateful for any help with this vexing problem. The ads are destroying one of the simple pleasures of my life.