Where was the folder before?

This article is a bit old now, but you could try it: Restore the default Smart Bookmarks Folders.

Any luck?

If you need to create your own Recently Bookmarked folder -- a "smart bookmark folder" that shows your most recently added bookmarks -- there is a way to do it.

Custom Recently Bookmarked List

Firefox allows query folders. These are not like normal folders, but instead show the results of a search across your bookmarks or history. "Most Visited" is an example: it shows the results of a search across your history.

To add a "Last 50 Bookmarks" Smart Folder:

(1) Copy this query string:

place:queryType=1&sort=12&maxResults=50&excludeQueries=1

I have bolded the part that specifies how long you want your list to be.

(2) Open the Library window ("Show All Bookmarks" or Ctrl+Shift+b)

This will display the Other Bookmarks category by default, which is fine.

(3) Right-click a blank area on the right and choose New Bookmark. In the Name, you can put something like Last50. Then paste the query string into Location and click Add.

(4) Drag the Last 50 "bookmark" to the Bookmarks Toolbar (if you use that, or the Bookmarks Menu). When you drop it, the icon should change from the "globe" to the "gear" indicating that Firefox has recognized it as a query.

(5) When you switch over the Firefox and view the folder, it should have the last 50 bookmarks you added.

I am attaching 2 screenshots illustrating.