// This is important: we need to ensure the 'inner' DOM window is created.
// If we don't, it is initialized while XPConnect is walking the outer scope
// chain, which reinitializes the JS context.  This causes JS_SetOptions to be
// called in the middle of execution, disabling our DONT_REPORT_UNCAUGHT flag,
// which prevents exceptions from propagating out.
nsCOMPtr<nsIDOMWindow> domWindow;
rv = webBrowser->GetContentDOMWindow(getter_AddRefs(domWindow));
if (NS_FAILED(rv)) {
    throw GeckoError("Failed to get content DOM window");
}
nsCOMPtr<nsPIDOMWindow> outer(do_QueryInterface(domWindow));
nsCOMPtr<nsPIDOMWindow>(outer->EnsureInnerWindow());

Run this code before calling JS_SetOptions.