Google: SEO for Firefox plugin, install it, turn it on, go to a forum of your choosing, look at other people's posts... if their links are highlighted in red, the links you can post are nofollow. If not, post away
change the css ff uses as standard and you dont need any plugin just make markup that strikes theough or colors any part with nofollow
I found this one http://allno.com/ It shows href, Anchor, nofollow, Head Meta but you have to enter URL