diff --git a/accessible/ipc/DocAccessibleParent.h b/accessible/ipc/DocAccessibleParent.h
index d28e0aa85963..ef1e9937d905 100644
--- a/accessible/ipc/DocAccessibleParent.h
+++ b/accessible/ipc/DocAccessibleParent.h
@@ -111,7 +111,7 @@ public:
    */
   void RemoveChildDoc(DocAccessibleParent* aChildDoc)
   {
-    aChildDoc->Parent()->SetChildDoc(nullptr);
+    aChildDoc->Parent()->ClearChildDoc(aChildDoc);
     mChildDocs.RemoveElement(aChildDoc);
     aChildDoc->mParentDoc = nullptr;
     MOZ_ASSERT(aChildDoc->mChildDocs.Length() == 0);
diff --git a/accessible/ipc/ProxyAccessibleBase.cpp b/accessible/ipc/ProxyAccessibleBase.cpp
index 67b76b0b45ec..8f1a2b7eda2e 100644
--- a/accessible/ipc/ProxyAccessibleBase.cpp
+++ b/accessible/ipc/ProxyAccessibleBase.cpp
@@ -52,15 +52,31 @@ ProxyAccessibleBase<Derived>::Shutdown()
 
 template <class Derived>
 void
-ProxyAccessibleBase<Derived>::SetChildDoc(DocAccessibleParent* aParent)
+ProxyAccessibleBase<Derived>::SetChildDoc(DocAccessibleParent* aChildDoc)
 {
-  if (aParent) {
-    MOZ_ASSERT(mChildren.IsEmpty());
-    mChildren.AppendElement(aParent);
-    mOuterDoc = true;
+  // DocAccessibleParent::AddChildDoc tolerates replacing one document with
+  // another. We must reflect that here.
+  MOZ_ASSERT(aChildDoc);
+  MOZ_ASSERT(mChildren.Length() <= 1);
+  if (mChildren.IsEmpty()) {
+    mChildren.AppendElement(aChildDoc);
   } else {
-    MOZ_ASSERT(mChildren.Length() == 1);
-    mChildren.Clear();
+    mChildren.ReplaceElementAt(0, aChildDoc);
+  }
+  mOuterDoc = true;
+}
+
+template <class Derived>
+void
+ProxyAccessibleBase<Derived>::ClearChildDoc(DocAccessibleParent* aChildDoc)
+{
+  MOZ_ASSERT(aChildDoc);
+  // This is possible if we're replacing one document with another: Doc 1
+  // has not had a chance to remove itself, but was already replaced by Doc 2
+  // in SetChildDoc(). This could result in two subsequent calls to
+  // ClearChildDoc() even though mChildren.Length() == 1.
+  MOZ_ASSERT(mChildren.Length() <= 1);
+  if (mChildren.RemoveElement(aChildDoc)) {
     mOuterDoc = false;
   }
 }
diff --git a/accessible/ipc/ProxyAccessibleBase.h b/accessible/ipc/ProxyAccessibleBase.h
index 62d6287b68a4..bea669ffac53 100644
--- a/accessible/ipc/ProxyAccessibleBase.h
+++ b/accessible/ipc/ProxyAccessibleBase.h
@@ -78,7 +78,8 @@ public:
 
   void Shutdown();
 
-  void SetChildDoc(DocAccessibleParent*);
+  void SetChildDoc(DocAccessibleParent* aChildDoc);
+  void ClearChildDoc(DocAccessibleParent* aChildDoc);
 
   /**
    * Remove The given child.