If 404 error related message is not put in the HTML title tag of the missing page, extension will not automatically look for an archive, but user can click on 'Archive' button instead. Clicking on that button will show archived version of any page.404 : Page is Not Found ? Now it will be! will search archives to see what happend to the page if it's not found.'False detection' occurs when an existing page is about this error and has error message in HTML title tag. However, these pages are not common. When this happens, just click 'Cancel' when confirm box appears, and that page will be added to 'whitelist' and will not be mistaken again for missing page.Requirements:

Ссылка: http://releases..b/mozilla.org/addons/4693/404___page_is_not_found___now_it_will_be!_-1.2.1-fx.xpi