In the way the Chrome browser manages bookmarks, this does it the same way. They are all within a single page, in a hierarchy that allows a drill out to a right-pane showing sub folders or to the URLs within the folders. This is a very easy and elegant way to manage bookmarks and I have been hoping either Mozilla or an extension would provide it. It also serves as a way to access all your bookmarks so you can hide the book mark toolbar and gain some vertical space: just pin the Book Mark Manager tab. I gave it 4 stars because it does not appear to support drag-drop of a URL from the address bar which is the only way I create new bookmarks and a feature supported in Chrome.
Response to developer's response- I don't see 5 stars on the right of the address bar but do see see three dots which list a variety of options. One option is to bookmark the page but there is no ability to control where it is saved in your book mark folders.

Nice , clean, and easy to use bookmark manager. I like simplicity.. not features and bloat I will never use.. This allows me to clean and move my bookmarks easily.

Looks good, but it would be nicer if the URL would change to reflect the path in the bookmarks. Then it would be possible to set a specific folder as homepage.

Update: thanks for implementing this 😍 I think it's a very neat looking extension 👍 One more feature I'm hoping for is tree-like expansion of sub-folders. It's already in the sidebar with the triangles, but not in the main view.

Looks pretty nice.