You can restore all the sites you "Dismiss" from the new tab page by deleting '''browser.newtabpage.blocked''' in [https://support.mozilla.org/en-US/kb/about-config-editor-firefox about:config]. Note that the addresses are encrypted in this setting for privacy. If you deleted the site from history then recovery may be impossible, a database recovery tool would need to be used on the '''places.sqlite''' file in your [[Profiles - Where Firefox stores your bookmarks, passwords and other user data|profile folder]].