Open the Customise page (right-click the 3 bar menu or a toolbar and select Customise Toolbar). Do you see the icon you want there? Perhaps the icon labelled Library? If so, drag it to the toolbar. If it isn't there either, try pressing the button at the bottom right of that page labelled Restore Defaults.

https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

You can drag the Bookmarks Menu button that shows the bookmarks in a drop-down list from the Customize palette to the Navigation Toolbar. This Bookmarks Menu button has a "View bookmarks sidebar" menu item.

See "How do I add the Bookmarks Menu button to the toolbar?"

• https://support.mozilla.org/en-US/kb/bookmarks-firefox