From MozillaZine Knowledge Base
To enable pop-up blocking in the Mozilla Suite:
- Click the Edit menu.
- Choose Preferences.
- Double-click the category you want to configure in the left-hand pane. In this case, double-click Privacy and Security.
- Click "Pop-up Blocking" in that same pane.
- Check "Block Pop-up Windows".
In older versions of the Suite, the option is in "Scripts and Plugins" to "Block Unrequested Pop-up Windows".
Other possible spellings: popups, popup blocker.