Restart Firefox.

Once Firefox processes the request, the preference will be changed again. Are the folders reappearing where you expect them? If not, it's possible they got moved into a subfolder. Unfortunately, it is difficult to find them because the search box in the Library dialog doesn't find Smart Bookmark Folders.

But you can search for them in an HTML-format backup. Here's how:

(1) Export Bookmarks to an HTML-format file that you save in a convenient location, such as your desktop. See: Export Firefox bookmarks to an HTML file to back up or transfer bookmarks

(2) Open that HTML file in Firefox and you can use Find (Ctrl+f) to look for the missing Smart Bookmark Folder. Then you can open that location in the Library and move it back where it belongs.

Any luck?

I did find the missing folder, "Recent Bookmarks". THANK YOU!

The change I make to about:config per

https://support.mozilla.org/en-US/kb/restore-default-smart-bookmarks-folders

do not "stick". When I close FF then re-open, the changed value has reset to the apparent default value of 7.

Guess it's a moot point as my immediate problem has been solved. Again Thanks.

The only purpose of resetting this pref is to make Firefox restore those smart bookmarks. The normal use of this pref is to store the browser.places.smartBookmarksVersion value to make sure that the folders are only created once and not on every start (you wouldn't want an extra folder on each start).