The "Forget about this site" feature is very literal: the entire site, all pages, everything Firefox knows, even your bookmarks on the site.

Because the Most Visited list (the blue "smart folder" list) is based on historical visits, and that data was erased, there's no easy way to add the site back. Probably it's best to add a new bookmark for it until it makes it way back into the top 10.

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't recover from this 'forget' unless you have a backup of the involved files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

Firefox keeps JSON backups of the bookmarks in the bookmarkbackups folder, one each day that Firefox has been started up to a maximum of 10.

• Bookmarks > Show All Bookmarks > Import & Backup > Restore > Choose File
• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox

Restoring a JSON backup will replace all existing bookmarks, so you lose bookmarks that were created at a time after this backup has been created.

Note that the xx part in the file name of the JSON backup (bookmarks-####-##-##_xx.json) shows the number of bookmarks stored in the backup.