mike's comment for userContextId

mike's comment for userContextId

diff --git a/browser/components/sessionstore/SessionStore.jsm b/browser/components/sessionstore/SessionStore.jsm
index b42440d..5ae8cce 100644
--- a/browser/components/sessionstore/SessionStore.jsm
+++ b/browser/components/sessionstore/SessionStore.jsm
@@ -2919,21 +2919,19 @@ var SessionStoreInternal = {
 
     let numVisibleTabs = 0;
 
     for (var t = 0; t < newTabCount; t++) {
       // When trying to restore into existing tab, we check if the userContextId
       // is matched, otherwise we create a new tab for restoring.
       let reuse = false;
       if (t < openTabCount) {
-        // If the tab (winData.tabs[t]) doesn't have userContextId, the value
-        // will be undefined, but getAttribute("usercontextid") will be "", so
-        // we use `|| ""` to let it has a default value "".
-        let userContextId = winData.tabs[t].userContextId || "";
-        reuse = tabbrowser.tabs[t].getAttribute("usercontextid") == userContextId;
+        let userContextId = winData.tabs[t].userContextId;
+        reuse = !userContextId ||
+                tabbrowser.tabs[t].getAttribute("usercontextid") == userContextId;
       }
 
       let tab;
       if (reuse) {
         tab = tabbrowser.tabs[t];
       } else {
         // There will be 2 cases when we try to create a new tab:
         // 1. When we have more tabs then existing tabs. i.e. t > openTabCount.