First, make sure the button is set in a toolbar. Right click on an empty part of the tab bar and choose "customise". If a "bookmarks" button is in the window that appears, drag it into a toolbar and it will be there.
Note: there's two versions. One works as a menu, one opens the bookmarks sidebar.
If the button is actually in a toolbar but it doesn't appear outside customise mode, then it's because some themes delete it (I never understood why). For me, what worked to fix it was finding the userChrome.css file and adding this line:
You can toggle the Bookmarks sidebar on/off via "View > Sidebars > Bookmarks" or Ctrl+B
You can find toolbar buttons to open the bookmarks (star) and the history (clock) in the sidebar in the toolbar palette and drag them on a toolbar.
There are two bookmark buttons with a star in the Customize window.
One star button has a drop marker that open a Bookmark menu.
The other star button without the drop marker opens the bookmarks in the sidebar.
Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible:
Firefox > Options
"View > Toolbars" (press F10 to display a hidden menu bar)
Open the Customize window to set which toolbar items to display:
Firefox > Options > Toolbar Layout
View > Toolbars > Customize
Check that "Bookmarks Toolbar items" is on the Bookmarks Toolbar
If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar
If missing items are in the toolbar 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 toolbar palette then click the "Restore Default Set" button to restore the default toolbar set up