//This code is placed in the root document header
//This function is written to run from the sandbox. It opens a url in an external browser
function openExternalURL(href) {
    var request = new air.URLRequest(href);
    try {            
        air.navigateToURL(request);
    }
    catch (e) {
        // handle error here
    }
}

//Create an object that will be used to expose AIR functionality to the browser sandbox
var Exposed = new Object();

//Exposed can now be read from the sandbox, so can the above function. The open url
//function is added as a method of the exposed object.
Exposed.openExternalURL = openExternalURL;

function doLoad() {
    //Place the Exposed object on the parentSandboxBridge property of the ui frame's window object.
    var frame = document.getElementById('UI').contentWindow.parentSandboxBridge = Exposed;
}
//doLoad is placed inline on the body tag.

//Now to run the external function from inside the sandbox use. The function expects
//an argument that contains a URL.
parentSandboxBridge.openExternalURL(href);

Adobe Air has a few security settings that stops you executing certain AIR methods when inside the sandbox. The code below bridges between the root document and the security sandbox, allowing you to open a URL in AIR in a default browser.

Adobe AIR Open New URL with Sandbox Bridge

url

JavaScript