"Forget About This Site" removed all past history visits and you can only recover if you have an older copy of places.sqlite (this file stores bookmarks and history). If you revisit this website then Firefox will start to save new history, so this only affects past history. ----- Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note. You can't recover from this 'forget' unless you have a backup of involved files. Note that since 110, passwords are no longer included just like bookmarks. *stop deleting saved passwords when user runs History > Forget this site command [702925] If you revisit a 'forgotten' website, data for that website will be saved once again. *https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox