This is something I noticed a few times with trunk. After I resume my Mac from sleep, Firefox is in a strange state where no page can be loaded any more. If you open a new tab and type an URL, nothing will be loaded and the content area will be kept blank (no Offline error page or console error). The File > Work Offline menu item is checked, however Components.classes["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService).offline returns false. If I click on "Work Offline", the menu item is still checked and Firefox enters true Offline mode: nsIIOService.offline is true and I get the Offline error pages when loading something. I can't reproduce this by just turning off network, seems to happen only when getting out of sleep, but not always.