Here's how you stop annoying popup windows from appearing in Firefox.
1. Go to Preferences in Firefox.
2. Go to the Content tab.
3. Make sure the checkbox next to "Block pop-up windows" is checked.
4. To allow a particular site to display popup windows, click the Exceptions box.
5. Enter the URL of the site you want to allow popups, then click the Allow button.
Now, all popup windows will be blocked except for the sites you specified in Exceptions.