For me, this is a "Must Have".

I have a lot of bookmarks, cleanly categorised in folders, subfolders and so on.

Using the built-in bookmarks menu, you are navigating down the hierarchy, and because you are impatient, you are moving the pointer fast. When you have almost reached the desired subfolder, you move the pointer a little bit outside the menu which immediately collapses and you have to start over.
This extension gives you a tree view which doesn't have that issue. Furthermore, the last opened folder is still open when you reopen the tree view.

Also creating a new folder for a new bookmark is much more convenient: While FF merely shows you a small control with expanded folders, the tree view is much bigger and lets you collapse all folders.