diff --git a/browser/components/sessionstore/content/content-sessionStore.js b/browser/components/sessionstore/content/content-sessionStore.js
index 71a392c1c47d..d33c7b75e15f 100644
--- a/browser/components/sessionstore/content/content-sessionStore.js
+++ b/browser/components/sessionstore/content/content-sessionStore.js
@@ -60,7 +60,7 @@ function createLazy(fn) {
  */
 function isSessionStorageEvent(event) {
   try {
-    return event.storageArea == content.sessionStorage;
+    return event.storageArea == event.target.sessionStorage;
   } catch (ex if ex instanceof Ci.nsIException && ex.result == Cr.NS_ERROR_NOT_AVAILABLE) {
     // This page does not have a DOMSessionStorage
     // (this is typically the case for about: pages)
diff --git a/browser/components/sessionstore/test/browser_sessionStorage.js b/browser/components/sessionstore/test/browser_sessionStorage.js
index 289bb15941d6..4afd9e0551b7 100644
--- a/browser/components/sessionstore/test/browser_sessionStorage.js
+++ b/browser/components/sessionstore/test/browser_sessionStorage.js
@@ -29,7 +29,17 @@ add_task(function session_storage() {
   is(storage["http://mochi.test:8888"].test, OUTER_VALUE,
     "sessionStorage data for mochi.test has been serialized correctly");
 
-  // Ensure that modifying sessionStore values works.
+  // Ensure that modifying sessionStore values works for the inner frame only.
+  yield modifySessionStorage2(browser, {test: "modified1"});
+  TabState.flush(browser);
+
+  ({storage} = JSON.parse(ss.getTabState(tab)));
+  is(storage["http://example.com"].test, "modified1",
+    "sessionStorage data for example.com has been serialized correctly");
+  is(storage["http://mochi.test:8888"].test, OUTER_VALUE,
+    "sessionStorage data for mochi.test has been serialized correctly");
+
+  // Ensure that modifying sessionStore values works for both frames.
   yield modifySessionStorage(browser, {test: "modified"});
   yield modifySessionStorage2(browser, {test: "modified2"});
   TabState.flush(browser);