If one wishes to point someone to a URL, it is sometimes useful to mark that URL as being unsafe for viewing in a workplace or any other location where viewing certain web pages might be inappropriate. By replacing the http in the URL with nsfw we achieve this.
This extension, which should work with most versions of Firefox, teaches it to understand the nsfw URL scheme. Visiting a URL such as nsfw://www.chiark.greenend.org.uk/ will bring up an interstitial page asking the user if they are sure they wish to visit the site in question.