How do I undo"forget this site" in history, if hit by accident

Sorry to say this, but, there is no undo function for Forget About This Site. That removes not only the history pages, but any bookmarks you had for the site, custom permissions/site exceptions, cookies, etc. How critical is the history? There might be a way to scrounge up an older list if you have a backup of your Firefox profile folder or if Windows created a shadow copy in a restore point recently. I suggest this tool for checking in restore points: http://nicbedford.co.uk/software/systemrestoreexplorer/ If you find a copy of places.sqlite that would be recent enough to be useful, you could restore it somewhere convenient such as your Documents folder (don't overwrite the current file), and then use one of these tools to inspect it: * http://www.nirsoft.net/utils/mozilla_history_view.html - this program can export history entries to an HTML file, which hopefully would give you links you to revisit MSN pages listed in the old places.sqlite file, if that's what you're looking for * https://addons.mozilla.org/firefox/addon/norwell/ - this add-on's description says that it can import from a places.sqlite file but I haven't tested that