Firefox Ad...
It's not an advertisement for Firefox. Craigslist uses "referrers" to keep tabs on what city you clicked to search. It writes a "cookie" into your browser cache, and that's why every time you return to Craigslist it sends you to the last city you looked in.
IE, Safari, and many other browsers will not allow you to disable the "referrer" since it's a pretty commonly used tracking method for many websites. Most shopping carts won't work with out them for example.
Firefox on the other hand will allow you to shut them off. Since it runs on literally any platform on the planet, that's why the website recommends it. Am I promoting Firefox? No. I personally don't like it one bit, however I do use it on my Windows laptop because I like IE even less!