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Tap the Firefox app icon, which resembles an orange fox wrapped around a blue globe. It's at the top of the Firefox screen. Type in about:config and then tap Search or ⏎ Return in the keyboard. If there's any text in the search bar, remove it before typing in about:config. You'll find this below the search bar at the top of the screen. Type in dom.disable_open_during_load and wait for the dom.disable_open_during_load option to appear. Tap the dom.disable_open_during_load section to expand it. You should see the pop-up blocker's status (which should be "true") listed on the left side of the screen. If the status says "false" instead, your Firefox browser is already blocking pop-ups. It's in the bottom-right corner of the pop-up blocker's section. Doing so will change the pop-up blocker's status from "true" to "false", signifying that the pop-up blocker is enabled. Even with the pop-up blocker enabled, not all pop-ups will be blocked.
Open Firefox. Tap the search bar. Go to the configuration page. Tap the "Search" text field. Search for the pop-up blocker. Select the pop-up blocker. Tap Toggle.