Till now when you reset Firefox -available when you select Troubleshooting Information on Help menu or visit about:support page- to its initial state it leaves Old Firefox Data folder on desktop which is your old profile contains important browser data such as bookmarks, extensions, history, passwords etc.  Mozilla has done this to let you restore that data if you want which no longer the case as it now cleans that old data automatically after reset.

Firefox clean old data

We don’t know from which version this change came into effect, so Firefox no longer saves old Firefox data folder to desktop and you don’t need to delete that folder either.