A Firefox add-on that create adds a new option in your Bookmarks menu
Version: 0.5.8Add Bookmark Here allows you to add bookmarks to any folder in the bookmarks menu like in Opera.
Operating System: Mac OS X
Add Bookmark Here adds an option to the bookmarks menu and/or the context menu for bookmark folders that enables you to add a bookmark to that folder.
This feature is present in Opera where it is called "Add page here..." and I think it's a very convenient way to keep the bookmarks sorted.