diff --git a/browser/components/sessionstore/test/browser/browser_248970_a.js b/browser/components/sessionstore/test/browser/browser_248970_a.js
index 2c2cb9b67ae0..ab8bc2939eb7 100644
--- a/browser/components/sessionstore/test/browser/browser_248970_a.js
+++ b/browser/components/sessionstore/test/browser/browser_248970_a.js
@@ -82,61 +82,65 @@ function test() {
     this.removeEventListener("load", arguments.callee, true);
 
     let prePBModeTimeStamp = getSessionstorejsModificationTime();
-    // enter private browsing mode
-    pb.privateBrowsingEnabled = true;
-    ok(pb.privateBrowsingEnabled, "private browsing enabled");
 
-    // sessionstore.js should be modified at this point
-    /* XXX this strangely fails!
-       TODO filed bug 462986
-    isnot(prePBModeTimeStamp, getSessionstorejsModificationTime(),
-      "sessionstore.js should be modified when entering the private browsing mode");
-    */
+    // make sure that the timestamp is bound to differ if sessionstore.js
+    // is modified when entering the private mode.
+    let timer = Cc["@mozilla.org/timer;1"].
+                createInstance(Ci.nsITimer);
+    timer.initWithCallback(function (aTimer) {
+      // enter private browsing mode
+      pb.privateBrowsingEnabled = true;
+      ok(pb.privateBrowsingEnabled, "private browsing enabled");
 
-    // record the time stamp of sessionstore.js in the private session
-    let startPBModeTimeStamp = getSessionstorejsModificationTime();
+      // sessionstore.js should be modified at this point
+      isnot(prePBModeTimeStamp, getSessionstorejsModificationTime(),
+        "sessionstore.js should be modified when entering the private browsing mode");
 
-    // private browsing session, add new tab: (B)
-    const testURL_B = "http://test1.example.org/";
-    let tab_B = gBrowser.addTab(testURL_B);
+      // record the time stamp of sessionstore.js in the private session
+      let startPBModeTimeStamp = getSessionstorejsModificationTime();
 
-    tab_B.linkedBrowser.addEventListener("load", function (aEvent) {
-      this.removeEventListener("load", arguments.callee, true);
+      // private browsing session, add new tab: (B)
+      const testURL_B = "http://test1.example.org/";
+      let tab_B = gBrowser.addTab(testURL_B);
 
-      // private browsing session, add new tab: (C)
-      const testURL_C = "http://localhost:8888/";
-      let tab_C = gBrowser.addTab(testURL_C);
-
-      tab_C.linkedBrowser.addEventListener("load", function (aEvent) {
+      tab_B.linkedBrowser.addEventListener("load", function (aEvent) {
         this.removeEventListener("load", arguments.callee, true);
 
-        // private browsing session, close tab: (C)
-        gBrowser.removeTab(tab_C);
+        // private browsing session, add new tab: (C)
+        const testURL_C = "http://localhost:8888/";
+        let tab_C = gBrowser.addTab(testURL_C);
 
-        // private browsing session, close tab: (B)
-        gBrowser.removeTab(tab_B);
+        tab_C.linkedBrowser.addEventListener("load", function (aEvent) {
+          this.removeEventListener("load", arguments.callee, true);
 
-        // private browsing session, close tab: (A)
-        gBrowser.removeTab(tab_A);
+          // private browsing session, close tab: (C)
+          gBrowser.removeTab(tab_C);
 
-        // record the timestamp of sessionstore.js at the end of the private session
-        gPrefService.setIntPref("browser.sessionstore.interval", ss_interval);
-        gPrefService.setIntPref("browser.sessionstore.interval", 0);
-        let endPBModeTimeStamp = getSessionstorejsModificationTime();
+          // private browsing session, close tab: (B)
+          gBrowser.removeTab(tab_B);
 
-        // exit private browsing mode
-        pb.privateBrowsingEnabled = false;
-        ok(!pb.privateBrowsingEnabled, "private browsing disabled");
+          // private browsing session, close tab: (A)
+          gBrowser.removeTab(tab_A);
 
-        // compare timestamps: pre and post private browsing session
-        is(startPBModeTimeStamp, endPBModeTimeStamp,
-          "outside private browsing - sessionStore.js timestamp has not changed");
+          // record the timestamp of sessionstore.js at the end of the private session
+          gPrefService.setIntPref("browser.sessionstore.interval", ss_interval);
+          gPrefService.setIntPref("browser.sessionstore.interval", 0);
+          let endPBModeTimeStamp = getSessionstorejsModificationTime();
 
-        // cleanup
-        gPrefService.setIntPref("browser.sessionstore.interval", ss_interval);
-        gPrefService.clearUserPref("browser.privatebrowsing.keep_current_session");
-        finish();
+          // exit private browsing mode
+          pb.privateBrowsingEnabled = false;
+          ok(!pb.privateBrowsingEnabled, "private browsing disabled");
+
+          // compare timestamps: pre and post private browsing session
+          is(startPBModeTimeStamp, endPBModeTimeStamp,
+            "outside private browsing - sessionStore.js timestamp has not changed");
+
+          // cleanup
+          gPrefService.setIntPref("browser.sessionstore.interval", ss_interval);
+          gPrefService.clearUserPref("browser.privatebrowsing.keep_current_session");
+          finish();
+        }, true);
       }, true);
-    }, true);
+    }, 50, Ci.nsITimer.TYPE_ONE_SHOT);
   }, true);
 }