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Flat Bookmarks + History is a Firefox extension which makes the Bookmarks and History Sidebars a flat list view, instead of the traditional tree view.
As you know, the Bookmarks Sidebar shows your entire bookmarks as a tree view, however it has a problem that the deeply-nested bookmarks are less visible because of their deep indentations. Flat Bookmarks + History solves the problem with a flat list view and provides some major features:

• Quick access to the root folders: Bookmarks Toolbar, Bookmarks Menu and Unsorted Bookmarks
• Breadcrumb Navigation (a list of parent folders with drop-down menus showing sub-folders)
• 'Show in Sidebar' menu in the context menu of any types of folder