FireFox(FF) has had this style of saving bookmarks(BK) at least since I've been using it (Version 2.0), and I have 11.0 now, what version are you using?
What I usually do:
I'll right click on web page >Click on "Bookmark This Page"
on context menu, this pops up.
Then I click on the down arrow where it says Bookmarks Menu, and this shows.
Where you can pick a previously chosen Folder or click on Choose.
If you do pick Choose then it changes to this.
Where you can scroll to, and click on a different Folder or create a new one.
If you you mis-sort a BK or new folder you can drag and drop within the BK toolbar, but sometimes it can be frustrating with different folders opening and closing too rapidly for you to place them right so click on Bookmarks in the Toolbar >Show all Bookmarks, and a new window will open where you can organize at your own pace.