Unfortunately, I am not aware of a built-in setting for that.

There is a Recently Bookmarked list if you are looking for something recent that you can't spot (for example, you don't remember which folder it's in):

"3-bar" menu button > Bookmarks

For the main Bookmarks Menu, two thoughts:

(1) If you don't want to set your own sort order

There are a couple add-ons that will auto-sort bookmarks. I think in the following one, if you select "Date added" and check the box for "Reversed" and check the box for "Always keep sorted" then your items will always appear with the newest one first. But if you move things around yourself, there's a risk that will get reset the next time you add a bookmark.

https://addons.mozilla.org/firefox/addon/sort-bookmarks-webext/

(2) If you want the option to manually control the sort order

You can use the following extension to specify your preferred folder for saving new bookmarks AND saving new bookmarks at the top rather than the bottom.

https://addons.mozilla.org/firefox/addon/default-bookmark-folder/

Note: disable this extension if you perform an import or restore to avoid it moving all the bookmarks from their original locations to your preferred folder during these bulk operations.

Thank you... the extension you posted may be a potential solution, or at least a start. I'll have to play with it awhile. Again, thanks for the response!

I should have mentioned one other thing. If you are still on the page you bookmarked, you can move it to a folder by clicking the blue star icon in the address bar to call up the editor. There's also a checkbox on that panel to show the drop-down every time you add a new bookmark using Command+D.

Thank you, that may be helpful too... I'll check it out...