diff --git a/mail/base/content/messageDisplay.js b/mail/base/content/messageDisplay.js
index 58385ce5a4..5b7b43d511 100644
--- a/mail/base/content/messageDisplay.js
+++ b/mail/base/content/messageDisplay.js
@@ -338,15 +338,10 @@ MessageDisplayWidget.prototype = {
 
     // figure out if we're looking at one thread or more than one thread
     let selectedMessages = this.folderDisplay.selectedMessages;
-    let selectedIndices = this.folderDisplay.selectedIndices;
-    let dbView = this.folderDisplay.view.dbView;
-    let firstThreadId = dbView.getThreadContainingIndex(selectedIndices[0])
-      .getChildHdrAt(0).messageKey;
+    let firstThreadId = selectedMessages[0].threadId;
     let oneThread = true;
-    for (let i = 0; i < selectedIndices.length; i++) {
-      let threadId = dbView.getThreadContainingIndex(selectedIndices[i])
-        .getChildHdrAt(0).messageKey;
-      if (threadId != firstThreadId) {
+    for (let i = 0; i < selectedMessages.length; i++) {
+      if (selectedMessages[i].threadId != firstThreadId) {
         oneThread = false;
         break;
       }