That is the overflow area and Firefox moves buttons that do not fit on the Navigation Toolbar automatically to this area. As this is done automatically, all you can do to prevent this is you make the Firefox window as wide as possible. You can possibly move buttons to you are less important yourself to this area to have the most important buttons always visible. Note that you can consider to move buttons to other toolbars as all toolbars accept buttons.

If you have the separate search bar on the Navigation Toolbar then you can consider to use the location bar instead or move the search bar to the overflow are (you can still use the Ctrl+K shortcut).