All history items (browsing and download history, cookies, cache, active logins, passwords, saved form data, exceptions for cookies, images, pop-ups) for that site will be removed.
At the top of the Firefox window, click the Firefox button, go over to the History menu (For Windows XP: click the History menu) and select Show All History to open the Library window.
Search for the website you want to remove from your history by typing its name in the Search History field in the top-right corner and then pressing EnterReturn.
Then, in the search results, right-clickhold down the Ctrl key while you click on the site you want to remove, and select Forget About This Site.