Hmm, sorry, there is no undo for that.

Obviously that function removes all the history for those sites, but also permissions, bookmarks, and cookies. The only data that is backed up is bookmarks, but unless you had multiple pages bookmarked on the two sites, I'm not sure it's worth attempting a restore.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks and history and cookies and passwords and cache and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then 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.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

For the bookmarks there are automatically created compressed .jsonlz4 backups in the bookmarkbackups folder.

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore

For passwords and other specific data you could check for older copies of involved files as part of a system restore point. System Restore Explorer: http://nicbedford.co.uk/software/systemrestoreexplorer/