I'm not sure there is a checkbox for this on mobile Firefox. If you want to do it "under the covers" here is how: (1) Open a new tab to '''about:config''' (type about:config in the Awesomebar and tap the go arrow or tap Enter). ''You may want to bookmark this internal screen for easier future access.'' (2) In the search box, type '''dom''' and tap the magnifying glass (3) Scroll down to '''dom.disable_open_during_load''' and tap it. Then tap the "Toggle" button on the right side to switch it from true to false. Back in the other tab, try the site again (for example, maybe a reload will trigger the popup). What happens?