--- a/testing/marionette/assert.js
+++ b/testing/marionette/assert.js
@@ -154,17 +154,24 @@ assert.content = function(context, msg =
 assert.open = function(context, msg = "") {
   // TODO: The contentBrowser uses a cached tab, which is only updated when
   // switchToTab is called. Because of that an additional check is needed to
   // make sure that the chrome window has not already been closed.
   if (context instanceof browser.Context) {
     assert.open(context.window);
   }
 
-  msg = msg || "Browsing context has been discarded";
+  if (!msg) {
+    if (context instanceof browser.Context) {
+      msg = `Browsing context ${context.curFrameId} has been discarded`;
+    } else {
+      msg = "Browsing context has been discarded";
+    }
+  }
+
   return assert.that(ctx => ctx && !ctx.closed,
       msg,
       NoSuchWindowError)(context);
 };
 
 /**
  * Asserts that there is no current user prompt.
  *