I've never really got the hang of Firefox add-ons. I think it's great that I can completely customise my web-browsing experience, but customise to what? Right now, I can open pretty much any page on the web and read it - how much better can a browser really get?
That said, there's a lot to be said for this: YouTube Comment Snob. It filters out YouTube comments based on profanity, excess punctuation, bad spelling, all caps, no caps and various other offences. It's an open question as to whether that will leave anything at all.
Near-simultaneously, xkcd gives us this: