When work is slow, I must admit I surf a whole lot and spend tons of time here. But I don't get fired. You know why? Results. Even with all my goofing off, I deliver better results and high value to my employer, and they know it. And they also know that when the going gets tough, I put away the browser and get'er'dun. For instance, my recent absence was due to me working 10 hour days (and 4+ hours a day on weekends) to cover for a sudden vacancy in the business, since I am the only other person in the building who understands how to operate our commercial scheduling and billing software.
Now that we have a replacement and I've got her trained somewhat, I can goof off a little again, while I catch up all the stuff that didn't get done while it was hell time.
But, yes, we've fired people for facebooking instead of doing the work I gave them to do. That's the difference. If your work is done, if at the end of the day your goals are reached and you are a demonstrably valuable asset... surf away IMO.