--- a/editor/libeditor/HTMLEditRules.cpp
+++ b/editor/libeditor/HTMLEditRules.cpp
@@ -3539,16 +3539,21 @@ HTMLEditRules::TryToJoinBlocksWithTransa
       CheckForInvisibleBR(*leftBlock, BRLocation::beforeBlock,
                           leftBlockChild.Offset());
     EditActionResult ret(NS_OK);
     if (mergeLists) {
       // XXX Why do we ignore the result of MoveContents()?
       int32_t offset = leftBlockChild.Offset();
       EditActionResult retMoveContents =
         MoveContents(*rightList, *leftList, &offset);
+      if (NS_WARN_IF(retMoveContents.Rv() == NS_ERROR_EDITOR_DESTROYED)) {
+        return ret;
+      }
+      NS_WARNING_ASSERTION(retMoveContents.Succeeded(),
+        "Failed to move contents from the right list to the left list");
       if (retMoveContents.Handled()) {
         ret.MarkAsHandled();
       }
       // leftBlockChild was moved to rightList.  So, it's invalid now.
       leftBlockChild.Clear();
     } else {
       // Left block is a parent of right block, and the parent of the previous
       // visible content.  Right block is a child and contains the contents we

--- a/editor/libeditor/HTMLEditRules.h
+++ b/editor/libeditor/HTMLEditRules.h
@@ -318,18 +318,19 @@ protected:
    * MoveContents() moves the contents of aElement to (aDestElement,
    * aInOutDestOffset).  DTD containment rules are followed throughout.
    *
    * @param aInOutDestOffset        updated to point after inserted content.
    * @return                        Sets true to handled if this actually moves
    *                                the nodes.
    *                                canceled is always false.
    */
-  EditActionResult MoveContents(Element& aElement, Element& aDestElement,
-                                int32_t* aInOutDestOffset);
+  MOZ_MUST_USE EditActionResult
+  MoveContents(Element& aElement, Element& aDestElement,
+               int32_t* aInOutDestOffset);
 
   /**
    * DeleteElementsExceptTableRelatedElements() removes elements except
    * table related elements (except <table> itself) and their contents
    * from the DOM tree.
    *
    * @param aNode               If this is not a table related element, this
    *                            node will be removed from the DOM tree.