You can only have the bookmarks in the sidebar. The Bookmarks Manager (Library) is always a standalone window.

• https://support.mozilla.org/en-US/kb/use-firefox-sidebar-access-bookmarks-history-synced

"Ctrl+B" is the keyboard shortcut to toggle the Bookmarks Sidebar. "Ctrl+Shift+B" is the keyboard shortcut to toggle the Bookmarks Toolbar. "Ctrl+Shift+O" is the keyboard shortcut to Open the Library.

rsterczek, also Firefox will start with the bookmarks side panel open or closed depending on which state it was in in the last window to be closed in the previous Firefox session.

So if you launch Firefox by opening a single window, then open the side panel, and then quit Firefox (via, for example, choosing Exit from the File menu), then the next time you start Firefox it should start with the bookmarks side panel already open.