No-NSFW is a Firefox Extension which warns you about NSFW links before you click them.
When you hover on an external link, the plugin gets the rating (NSFW, SFW or unrated) for that link, which is displayed on the bottom-right of the screen as a small unintrusive warning.
No-NSFW analyses the domain, URL, title keywords and user votes to determine the page rating.
Windows 2000/XP/Vista, Mozilla Firefox 1.5 – 2.0