Try the Ctrl+B shortcut.

But also, look for the Sidebars button on the Customize screen. There's typically a box of available buttons you can drag-and-drop to your toolbar. See: Customize Firefox controls, buttons and toolbars.

Were you able to get the button back?

Well, also, View Bookmarks Sidebar is a command available on the optional bookmarks menu button for the toolbar (). It's another button you can add using the Customize panel.