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On Android devices, the Firefox start page is actually a grid showing previews of your "top sites." To view this start page, tap on the title bar, then Bookmarks, then Firefox Start. On the start page, tap and hold on the site you want to add permanently. Select Pin site from the pop up menu to make it a permanent fixture of your start page. If you don't see the site you want on the grid, tap and hold a square you are not interested in. This time, select Edit from the pop-up menu. Now you can enter a URL, or choose a site from your bookmarks or most visited sites. If you swipe out of the Firefox app, it will continue running in the background. If you want to see your pinned sites the next time you open the app, tap the menu icon and select Quit.
Visit the Firefox start page. Pin a site to your start page. Add a new site to your start page. Quit the app at the end of each session.