Yes restore from the latest bookmarks backup (.json file), they are stored in the bookmarkbackups folder in your profile but that should be transparent to you for the following steps.

1. "Ctrl+Shift+B" to bring up the bookmarks library list (Bookmarks > Show All Bookmarks)
2. On the library menu "Import and Backup" then ":Restore"
3. choose the last bookmarks backup from the list

All of your bookmarks will be replaced by the bookmark backup file, it will run incredibly fast.

Please mark "Solved" one answer that will best help others with a similar problem -- hope this was it.

If you haven't closed Firefox then you can also try Organize > Undo (Ctrl+Z) in the Library (Bookmarks Manager)

Be aware that "Most Visited" and "Recently Bookmarked" Smart Bookmarks folders and are not real existing folders.

• Such smart folders show a list created by a query of the places.sqlite database that stores the bookmarks and the history in Firefox.
• Smart folder lists show a maximum of 10 entries by default.
• Bookmarks that show up in a smart folder list are stored elsewhere in another folder and any changes made are applied to the real bookmark or history item.
• If you remove an entry then the list is shifted up and a new entry that wasn't shown previously is added to show 10 entries.
• If you add a new bookmark or visit a website then a new item is added at the top and the entry at the bottom disappears from that list.
• Items that disappear from the list are not gone, but merely do not show up anymore in that list.
• Actions like copy & paste or delete that you perform on bookmarks in such a list are done on the original bookmark.