diff --git a/mail/base/content/folderDisplay.js b/mail/base/content/folderDisplay.js
index 3390648bd4..0ab37f0f0b 100644
--- a/mail/base/content/folderDisplay.js
+++ b/mail/base/content/folderDisplay.js
@@ -2298,6 +2298,38 @@ FolderDisplayWidget.prototype = {
     });
   },
 
+  /**
+   * The maximum number of messages canMarkThreadAsRead will look through.
+   * If the number exceeds this limit, as a performance measure, we return
+   * true rather than looking looking through the messages and possible
+   * submessages.
+   */
+  MAX_COUNT_FOR_MARK_THREAD: 1000,
+
+  /**
+   * Check if the thread for the currently-selected message can be marked as
+   * read. A thread can be marked as read if and only if it has at least one
+   * unread message.
+   */
+  get canMarkThreadAsRead() {
+    let hasUnread = this.displayedFolder.getNumUnread(false) > 0;
+    if (this.displayedFolder && hasUnread) {
+      // If the messages limit is exceeded we bail out early and return true.
+      if (this.selectedIndices.length > this.MAX_COUNT_FOR_MARK_THREAD) {
+        return true;
+      }
+
+      for (let i of this.selectedIndices) {
+        if (
+          this.view.dbView.getThreadContainingIndex(i).numUnreadChildren > 0
+        ) {
+          return true;
+        }
+      }
+    }
+    return false;
+  },
+
   /**
    * @return true if all the selected messages can be deleted from their
    * folders, false otherwise.
diff --git a/mail/base/content/mail3PaneWindowCommands.js b/mail/base/content/mail3PaneWindowCommands.js
index 949b60d6b4..9caa8c586a 100644
--- a/mail/base/content/mail3PaneWindowCommands.js
+++ b/mail/base/content/mail3PaneWindowCommands.js
@@ -478,8 +478,9 @@ var DefaultController = {
       case "cmd_tag8":
       case "cmd_tag9":
       case "cmd_toggleRead":
-      case "cmd_markThreadAsRead":
         return gFolderDisplay.selectedCount > 0;
+      case "cmd_markThreadAsRead":
+        return gFolderDisplay.canMarkThreadAsRead;
       case "cmd_markAsRead":
         return CanMarkMsgAsRead(true);
       case "cmd_markAsUnread":
diff --git a/mail/base/content/messageWindow.js b/mail/base/content/messageWindow.js
index f6263e54f9..2e065eada8 100644
--- a/mail/base/content/messageWindow.js
+++ b/mail/base/content/messageWindow.js
@@ -443,6 +443,7 @@ function delayedOnLoadMessageWindow() {
   SetupCommandUpdateHandlers();
 
   gMessageDisplay = new StandaloneMessageDisplayWidget();
+  // eslint-disable-next-line no-global-assign
   gFolderDisplay = new StandaloneFolderDisplayWidget(gMessageDisplay);
   gFolderDisplay.msgWindow = msgWindow;
   gFolderDisplay.messenger = messenger;
@@ -1133,7 +1134,6 @@ var MessageWindowController = {
       case "cmd_tag8":
       case "cmd_tag9":
       case "button_mark":
-      case "cmd_markThreadAsRead":
       case "cmd_markReadByDate":
       case "cmd_viewAllHeader":
       case "cmd_viewNormalHeader":
@@ -1142,6 +1142,8 @@ var MessageWindowController = {
         return true;
       case "cmd_markAllRead":
         return false;
+      case "cmd_markThreadAsRead":
+        return gFolderDisplay.canMarkThreadAsRead;
       case "cmd_markAsRead":
         return CanMarkMsgAsRead(true);
       case "cmd_markAsUnread":
diff --git a/mail/test/browser/folder-display/browser_messageCommands.js b/mail/test/browser/folder-display/browser_messageCommands.js
index 53145606d2..2573c2c41c 100644
--- a/mail/test/browser/folder-display/browser_messageCommands.js
+++ b/mail/test/browser/folder-display/browser_messageCommands.js
@@ -308,6 +308,69 @@ add_task(function test_mark_all_read() {
   Assert.ok(allReadDisabled, "Mark All Read menu item should be disabled!");
 });
 
+add_task(function test_mark_thread_as_read() {
+  let unreadThreadFolder = create_folder("UnreadThreadFolder");
+  add_sets_to_folders([unreadThreadFolder], [create_thread(3)]);
+  be_in_folder(unreadThreadFolder);
+  make_display_threaded();
+
+  let serviceState = Services.prefs.getBoolPref(
+    "mailnews.mark_message_read.auto"
+  );
+  if (serviceState) {
+    // If mailnews.mark_message_read.auto is true, then we set it to false.
+    Services.prefs.setBoolPref("mailnews.mark_message_read.auto", false);
+  }
+
+  // Make sure Mark Thread as Read is enabled with >0 messages in thread unread.
+  right_click_on_row(0);
+  wait_for_popup_to_open(mc.e("mailContext"));
+  mc.click_menus_in_sequence(mc.e("mailContext"), [{ id: "mailContext-mark" }]);
+
+  let markThreadAsReadDisabled = mc.e("mailContext-markThreadAsRead").disabled;
+  Assert.ok(
+    !markThreadAsReadDisabled,
+    "Mark Thread as read menu item should not be disabled!"
+  );
+
+  // Make sure messages are read when Mark Thread as Read is clicked.
+  right_click_on_row(0);
+  wait_for_popup_to_open(mc.e("mailContext"));
+  mc.click_menus_in_sequence(mc.e("mailContext"), [
+    { id: "mailContext-mark" },
+    { id: "mailContext-markThreadAsRead" },
+  ]);
+  close_popup(mc, mc.eid("mailContext"));
+
+  let curMessage = select_click_row(0);
+  Assert.ok(curMessage.isRead, "Message should have been marked read!");
+
+  // Make sure Mark Thread as Read is now disabled with all messages read.
+  right_click_on_row(0);
+  wait_for_popup_to_open(mc.e("mailContext"));
+  mc.click_menus_in_sequence(mc.e("mailContext"), [{ id: "mailContext-mark" }]);
+
+  markThreadAsReadDisabled = mc.e("mailContext-markThreadAsRead").disabled;
+  Assert.ok(
+    markThreadAsReadDisabled,
+    "Mark Thread as read menu item should  be disabled!"
+  );
+
+  // Make sure that adding an unread message enables Mark Thread as Read once more.
+  curMessage.markRead(false);
+  right_click_on_row(0);
+  wait_for_popup_to_open(mc.e("mailContext"));
+  mc.click_menus_in_sequence(mc.e("mailContext"), [{ id: "mailContext-mark" }]);
+
+  markThreadAsReadDisabled = mc.e("mailContext-markThreadAsRead").disabled;
+  Assert.ok(
+    !markThreadAsReadDisabled,
+    "Mark Thread as read menu item should not be disabled!"
+  );
+
+  Services.prefs.setBoolPref("mailnews.mark_message_read.auto", true);
+});
+
 add_task(function test_shift_delete_prompt() {
   be_in_folder(shiftDeleteFolder);
   let curMessage = select_click_row(0);