Firefox also includes integrated customizable pop-up blocking. Firefox was given this feature early in beta development, and it was a major comparative selling point of the browser until Internet Explorer gained the capability in the Windows XP Service Pack 2 beta. This blocks pop-ups from all web sites by default, but can be configured to allow individual sites to show pop-ups. It can also be turned off entirely to allow pop-ups from all sites. Firefox’s pop-up blocking can be inconvenient at times — it prevents JavaScript-based links opening a new window while a page is loading unless the site is added to a “safe list” found in the options menu.

In many cases it is possible to view the pop-up’s URL by clicking the dialogue that appears when one is blocked. This makes it easier to decide if the pop-up should be displayed.