Hi, sites are added to History automatically when you visit them. To remove an entry, right click > Delete. To delete multiple sites, select Show All History at the bottom, then hold down Ctrl whilst selecting them, and right click > Delete.

If no history is being saved, make sure Firefox is set to remember history - Settings for privacy, browsing history and do-not-track

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If you mean the arrow at the far right then this list shows items you typed or pasted in the location/address bar sorted by frecency (visits bonus points) like the suggestions. There is no direct way to add items to this list at a specific position.