Seeing more popups recently despite Firefox's built-in popup blocking? Chances are they're using a Flash embed to sneak through. From the above site, here's a little-known setting to disable this trick:

1. Type about:config into the Firefox location bar.
2. Right-click on the page and select New and then Integer.
3. Name it privacy.popups.disable_from_plugins
4. Set the value to 2.