You can not undo that.

I'm *sure* there must be a way to undo it, since info about "forgetting" the site is stored in one of those damn SQLite files. I have tried using the SQLite browser to find it, but haven't been successful.

Surely someone knows, and maybe could provide the clues so that I could correct my "forgotten" bank login page that keeps wanting me to provide my grandfather's first name because Mozilla now won't store cookies for the "forgotten" web page?

There must be *someone* out there who helped write the "most visited" code who knows...?

Forget about this site will remove all the current content (e.g. bookmarks, cookies, passwords, cache, history, exceptions) that you have saved from a website. It doesn't have any lasting effects. If you (re)visit such a 'forgotten' website again then data from that site will be saved once again.

You can not recover the forgotten data unless you have a backup of the corresponding files in the Firefox Profile Folder

Be aware that "Forget About This Site" does more then deleting the history, so be careful with using it. Firefox will forget (remove) everything from the selected site and that includes passwords, cookies, saved form data, exceptions (images, cookies, pop-ups) as well as bookmarks and history and cache items.