Firefox only: Since the dawn of the Like button on Facebook, people have rallied for its counterpart, the Dislike button. Although Facebook hasn't stepped up to the plate with their own Dislike button, French developer Thomas Moquet has made it possible for Firefox users.
If your Facebook feed is anything like ours, a Dislike button could be more than useful to let the overly lewd, crude or socially unacceptable friend know just how you feel about their updates. (Then again, you could just hide that user.) Only other Firefox users who have the add-on installed will be able to see your disapproval, so it's not as though it's tapping into some secret Facebook feature or anything like that.
Dislike 0.2 is a free, experimental extension, and works wherever Firefox does.