That's been worked on at present. See: https://connect.mozilla.org/t5/discussions/firefox-sidebar-and-vertical-tabs-try-them-out-in-nightly/td-p/63231

You can only have the Bookmarks Toolbar placed at the top of the Firefox window, below the Navigation Toolbar. There are other ways to access the bookmarks and especially the Bookmarks Toolbar folder. "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. You can drag the Bookmarks Menu button [[Image:Bookmark Menu icon (star on tray)|width=16]] that shows the bookmarks in a drop-down list and drag the Library button that gives access to recent bookmarks and history from the Customize palette to the Navigation Toolbar. The 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 *https://support.mozilla.org/en-US/kb/use-firefox-sidebar-access-bookmarks-history-synced