gecko-dev/netwerk/test/unit/test_offline_status.js

function run_test() {
  var ioService = Cc["@mozilla.org/network/io-service;1"]
                    .getService(Ci.nsIIOService);

  try {
    var linkService = Cc["@mozilla.org/network/network-link-service;1"]
                        .getService(Ci.nsINetworkLinkService);

    // The offline status should depends on the link status
    Assert.notEqual(ioService.offline, linkService.isLinkUp);
  } catch (e) {
    // The network link service might not be available
    Assert.equal(ioService.offline, false);
  }
}