Many of the popup ads that can get around Firefox's popup blocker are Flash based instead of Java-script based. You can block these ads easily by changing a few settings.
1. Load Firefox and type in about:config in the address bar.
2. Right Click on a blank area of the screen and select New and then Integer.
3. For the window that says Enter the preference name, type in privacy.popups.disable_from_plugins
4. Click OK. Now you should see another window. Type in 2 and then click OK.
5. Those annoying popup ads should blocked from now on.