Advertisements on websites are annoying, often breaking up text and flashing to get your attention. To stop about 99% of the ads on the internet from even showing, add the following code to your userContent.css file:

I was finally sufficiently annoyed by obtrusive Flash advertisements at the Washington Post that I tracked down this information on hiding them from view.

In the course of working through this issue, I was also annoyed to find that the Washington Post inserts web bugs of the form

http://media.washingtonpost.com/wp-srv/google/tracker.gif?794673

on every story that it feeds. It's not exactly clear what they're doing with it, and why it's called a Google tracker, because they show up even when you're not coming from Google News.

Reliably Blocking (Flash) Advertisements in Gecko-based Browsers