You can't recover from this 'forget' unless you have a backup of the involved files. Using 'Forget About This Site' will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.

You can remove all data stored in Firefox from a specific domain via 'Forget About This Site' in the right-click context menu of an history entry ('History > Show All History' or 'View > Sidebar > History') or via the about:permissions page. Press 'Ctrl + F5' or press 'Ctrl + Shift + R' (Windows,Linux).Hold down the Shift key and left-click the Reload button.Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.