Mozilla offers a way to hide ads on web pages. It's not as easy as blocking pop-ups but if you are allergic to ads you should give it a try. The trick is to use the userContent.css file which allows users to customize the content that appears in the browser windows. Examples are poping up around the web that hide or dim (reducing their opacity) the ads:
floppymoose (hide) - more info
Jan Moesen (dim)
Adrian Holovaty (dim)
tidakada (hides Flash banners)
I'm sure there are a lot more variations on the same theme (add your favorites in a comment). So far there is one missing for what I find the most annoying ad practice today: blocking those Flash ads that are sliced just before the page you want to read (Salon is heavily using them, so now I hate going on Salon).
Here's my solution to Flash ads: I find the Flash plugin and drag it to the trash! This will work for a lot of folks, as most people simply don't need flash at all.