var proxies = ["PROXY myproxy.foo.com:9050", "PROXY myproxy.bar.com:8080",
"PROXY myproxy.baz.com:6667"], t1 = new Date().getTime(), counter = 1;

var secondsToSwitchAfter = 3;

function FindProxyForURL(url, host) {
  var t2 = new Date.getTime();
  if (t2 - t1 > secondsToSwitchAfter * 1000)
    counter++;
  t1 = t2;
  return proxies[counter % proxies.length];
}