diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index d8085b5d35f..da23f9e50d0 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -15918,27 +15918,36 @@ static uint32_t CountFullscreenSubDocuments(Document& aDoc) {
 }
 
 bool Document::IsFullscreenLeaf() {
-  // A fullscreen leaf document is fullscreen, and has no fullscreen
-  // subdocuments.
-  //
-  // FIXME(emilio): This doesn't seem to account for fission iframes, is that
-  // ok?
-  return Fullscreen() && CountFullscreenSubDocuments(*this) == 0;
+  // A fullscreen leaf document is fullscreen, and its fullscreen element does
+  // not embed another in-process fullscreen document, i.e. it is at the bottom
+  // of the fullscreen document chain. Other subdocuments may still be
+  // fullscreen without being part of that chain, for example when this document
+  // has more than one fullscreen element in its top layer.
+  Element* fsElement = GetUnretargetedFullscreenElement();
+  if (!fsElement) {
+    return false;
+  }
+
+  Document* subDoc = GetSubDocumentFor(fsElement);
+  if (!subDoc) {
+    return true;
+  }
+
+  return !subDoc->Fullscreen();
 }
 
 /* static */ Document* Document::GetFullscreenLeaf(Document& aDoc) {
   if (aDoc.IsFullscreenLeaf()) {
     return &aDoc;
   }
-  if (!aDoc.Fullscreen()) {
+  Element* fsElement = aDoc.GetUnretargetedFullscreenElement();
+  if (!fsElement) {
     return nullptr;
   }
-  Document* leaf = nullptr;
-  aDoc.EnumerateSubDocuments([&leaf](Document& aSubDoc) {
-    leaf = GetFullscreenLeaf(aSubDoc);
-    return leaf ? CallState::Stop : CallState::Continue;
-  });
-  return leaf;
+  Document* subDoc = aDoc.GetSubDocumentFor(fsElement);
+  MOZ_ASSERT(subDoc);
+  MOZ_ASSERT(subDoc->Fullscreen());
+  return GetFullscreenLeaf(*subDoc);
 }
 
 /* static */ Document* Document::GetFullscreenLeaf(Document* aDoc) {
@@ -15953,8 +15962,6 @@ bool Document::IsFullscreenLeaf() {
 
 static CallState ResetFullscreen(Document& aDocument) {
   if (Element* fsElement = aDocument.GetUnretargetedFullscreenElement()) {
-    NS_ASSERTION(CountFullscreenSubDocuments(aDocument) <= 1,
-                 "Should have at most 1 fullscreen subdocument.");
     aDocument.CleanupFullscreenState();
     NS_ASSERTION(!aDocument.Fullscreen(), "Should reset fullscreen");
     DispatchFullscreenChange(aDocument, fsElement);
@@ -16129,7 +16136,8 @@ void Document::RestorePreviousFullscreenState(UniquePtr<FullscreenExit> aExit) {
 
   Document* lastDoc = exitElements.LastElement()->OwnerDoc();
   size_t fullscreenCount = lastDoc->CountFullscreenElements();
-  if (!lastDoc->GetInProcessParentDocument() && fullscreenCount == 1) {
+  if ((!lastDoc->GetInProcessParentDocument() && fullscreenCount == 1) ||
+      GetFullscreenLeaf(lastDoc) != fullScreenDoc) {
     // If we are fully exiting fullscreen, don't touch anything here,
     // just wait for the window to get out from fullscreen first.
     PendingFullscreenChangeList::Add(std::move(aExit));
@@ -16929,8 +16937,6 @@ void Document::RemoteFrameFullscreenReverted() {
 
 static bool HasFullscreenSubDocument(Document& aDoc) {
   uint32_t count = CountFullscreenSubDocuments(aDoc);
-  NS_ASSERTION(count <= 1,
-               "Fullscreen docs should have at most 1 fullscreen child!");
   return count >= 1;
 }
 
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 82bbb321b84..9dffe87e101 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -2139,8 +2139,11 @@ class Document : public nsINode,
       UniquePtr<FullscreenExit>);
 
   /**
-   * Returns true if this document is a fullscreen leaf document, i.e. it
-   * is in fullscreen mode and has no fullscreen children.
+   * Returns true if this document is a fullscreen leaf document, i.e. it is
+   * in fullscreen mode and its current fullscreen element does not embed
+   * another in-process fullscreen document. Note that this document may still
+   * have other fullscreen subdocuments which are not part of the current
+   * fullscreen document chain.
    */
   bool IsFullscreenLeaf();