Find the element Frame.
Get ContentWindow
Convert to Document
Ex (getting first frame):
var lstFrame = geckoWebBrowser1.Document.GetElementsByTagName("FRAME");
if (lstFrame.Lenght < 1)
return;
var content= ((Gecko.DOM.GeckoFrameElement)lstFrame[0]).ContentWindow;
var document = content.Document as GeckoDocument;
sr = document.GetElementById("e-mailoutline-row-($Inbox)1");
.....