if (fastly.ff.visits_this_service == 0) {

  # Option 1: detect trailing slash and trigger client side redirect
  # From the client prespective, this means a request ending in a 
  # slash will elicit a redirect response.
  if (req.url ~ "/$") {
    error 600;
  }

  # Option 2: remove the trailing slash from the request
  # From the client prespective, this means the request works whether
  # or not it ends in a slash...
  set req.url = regsub(req.url, "/$", "");
}