Hi,

Usually there is no way to restore it unless you have a backup of the history + bookmarks file, places.sqlite which is located in the Firefox Profile Folder. You can try right-clicking on the file > Restore previous versions > Restore after exiting Firefox. If no previous versions are shown, you can also try going up one folder level and right-click the profile folder > Restore previous versions > Open or Copy to another location and try to copy/overwrite the file. Please note that the Restore feature may not be present on every Windows versions. You can also open the profile folder from Firefox Help (Alt + H) > Troubleshooting Information > Open Containing Folder, and exit Firefox.

Firefox Profile Folder & Files

Clearing History

Using "Forget About This Site" will remove everything from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious if you need to keep passwords or other data from that site.
You can't recover from that "forget" unless you have a backup of the affected 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.

To make the site (re)appear in most visited you will have to revisit the site often enough to make it match other entries in that most visited list.