var bInStream = Cc['@mozilla.org/binaryinputstream;1'].
                createInstance(Ci.nsIBinaryInputStream);
var fileInStream = Cc["@mozilla.org/network/file-input-stream;1"].
                   createInstance(Ci.nsIFileInputStream);

fileInStream.init(icon, MODE_RDONLY, PERMS_FILE, 0);
bInStream.setInputStream(fileInStream);

var byteArray = bInStream.readByteArray(bInStream.available());

// Convert the byte array to a base64-encoded string
var str = btoa(String.fromCharCode.apply(null, bytes));

engine._iconURI = makeURI(ICON_DATAURL_PREFIX + str);