See this support article - https://support.mozilla.org/en-US/kb/bookmarks-toolbar-display-favorite-websites

Make sure that toolbars like the "Bookmarks Toolbar" are visible. *"3-bar" Firefox menu button > Customize > Show/Hide Toolbars *View > Toolbars<br>Tap the Alt key or press F10 to show the Menu Bar *Right-click empty toolbar area Open the Customize window and set which toolbar items to display. *"3-bar" Firefox menu button > Customize *check that "Bookmarks Toolbar items" is on the Bookmarks Toolbar *if "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the Customize palette into the Customize window to the Bookmarks Toolbar *if missing items are in the Customize palette then drag them back from the Customize window on the toolbar *if you do not see an item on a toolbar and in the Customize palette then click the Restore Defaults button to restore the default toolbar setup *https://support.mozilla.org/kb/customize-firefox-controls-buttons-and-toolbars