The stock menu "Forget this site" in Firefox delete everything (including cookies, saved passwords, localStorage etc) of the selected website. This add-on add one context menu "Delete History of This Site" on history item and tab, it only delete the history of the selected website, keep you cookies, saved passwords, localStorage.
No Preferences UI
Only one about:config item:
to decide show confirm dialog while delete or not, it apply to all sites, you can set it to "true" to see the confirm dialog again.