This has been a popular topic of discussion on the Mozilla Support Forums:
Before Firefox quantum, when pressing Ctrl+D, the created bookmark was
put in the bookmarks menu by default (unlike clicking the star
bookmark button, which defaulted to unsorted bookmarks). After
updating to quantum, the default location is "Other bookmarks" - the
same as pressing the star button.
How do I restore the old behaviour?
Their moderators chimed in to say that there currently isn't a built-in way to restore the Ctrl+D functionality to its former state. However, they also suggested that you can restore the previous Ctrl+D capability with a relatively new add-on, Default Bookmark Folder:
Firefox built-in bookmarking:
This feature only affect bookmarks added through the built-in Firefox
bookmark icon or shortcut (Ctrl+D), or through the context menu.
- Select where the new bookmarks will be added
- Choose to add the new bookmarks to the top or the bottom of the selected folder