The open source web browser Mozilla Firefox features a number of tools for blocking web annoyances that include pop-up ads. By default, the pop-up blocker in Firefox is enabled but if you are still seeing pop-up ads, the tool might have been turned off at some point. You can quickly re-enable the pop-up blocker from the Firefox settings.
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Select "Firefox," then "Preferences" and then "Preferences."
Select the "Content" tab.
Check "Block pop-up windows" and then click "Close."