This add-on facilitates opening multiple tabs from the Bookmarks Menu and various other bookmark/history related dropdown menus. Less need to keep reopening menu, without using the sidebar. Optional auto-closing feature adds extra convenience.
Here is more about the add-on from the developers:
Bookmarks or History menu stays open when middle-clicking (or Ctrl-clicking) items, so you can easily open multiple items. Especially useful when you wish to view many, but not all, menu items in tabs (e.g. RSS feed items). Works similarly on the Bookmarks Toolbar, Bookmarks Button, and the Smart Location Bar. For extra convenience, turn on the auto-closing option, and the menu will automatically close after it detects that you’ve moved the mouse off the menu.
Want to learn more? Check out Stay-Open Menu on the Firefox Add-ons website.