The Library is the bookmarks and history manager with two panels. The left panel allows to select what yo view in the right panel. The right panel in the Library also has a detail pane at the bottom where you can inspect and modify details. You can sort the content in the right panel via the Views menu or by clicking a column header. You can export in import bookmarks as HTML, this only works in the Library for viewing purposes (i.e. no actual sorting is done). *https://support.mozilla.org/en-US/kb/bookmarks-firefox The sidebar is for easy access to bookmarks and history and synced tabs in the same page as where you open the website (i.e. no need to switch windows), but is smaller and you can only see the title of a bookmark and history (i.e. you need to hover to see the URL). *https://support.mozilla.org/en-US/kb/use-firefox-sidebar-access-bookmarks-history-synced