diff --git a/mail/base/content/about3Pane.js b/mail/base/content/about3Pane.js
index 133e98661f..3295edb464 100644
--- a/mail/base/content/about3Pane.js
+++ b/mail/base/content/about3Pane.js
@@ -2920,8 +2920,11 @@ var threadPane = {
     }
   },
 
+  /**
+   * Handle threadPane drag events.
+   */
   _onDragStart(event) {
-    let row = event.target.closest(`tr[is="thread-row"]`);
+    let row = event.target.closest(`tr[is^="thread-"]`);
     if (!row) {
       event.preventDefault();
       return;