diff --git a/dom/base/nsCopySupport.cpp b/dom/base/nsCopySupport.cpp
index 99b6af473f30..98e0e1c9247d 100644
--- a/dom/base/nsCopySupport.cpp
+++ b/dom/base/nsCopySupport.cpp
@@ -840,17 +840,20 @@ nsCopySupport::FireClipboardEvent(EventMessage aEventMessage,
     doDefault = (status != nsEventStatus_eConsumeNoDefault);
   }
 
+  // When this function exits, the event dispatch is over. We want to disconnect
+  // our DataTransfer, which means setting its mode to `Protected` and clearing
+  // all stored data, before we return.
+  auto clearAfter = MakeScopeExit([&] {
+    if (clipboardData) {
+      clipboardData->SetMode(DataTransfer::Mode::Protected);
+      clipboardData->ClearAll();
+    }
+  });
+
   // No need to do anything special during a paste. Either an event listener
   // took care of it and cancelled the event, or the caller will handle it.
   // Return true to indicate that the event wasn't cancelled.
   if (originalEventMessage == ePaste) {
-    // Clear and mark the clipboardData as readonly. This prevents someone
-    // from reading the clipboard contents after the paste event has fired.
-    if (clipboardData) {
-      clipboardData->ClearAll();
-      clipboardData->SetMode(DataTransfer::Mode::Protected);
-    }
-
     if (aActionTaken) {
       *aActionTaken = true;
     }
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index a7b9a8e4ec5b..d4988af9e5a8 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -1772,6 +1772,12 @@ EventStateManager::GenerateDragGesture(nsPresContext* aPresContext,
 
       RefPtr<DataTransfer> dataTransfer =
         new DataTransfer(window, eDragStart, false, -1);
+      auto protectDataTransfer = MakeScopeExit([&] {
+        if (dataTransfer) {
+          dataTransfer->SetMode(DataTransfer::Mode::Protected);
+          dataTransfer->ClearAll();
+        }
+      });
 
       nsCOMPtr<nsISelection> selection;
       nsCOMPtr<nsIContent> eventContent, targetContent;
@@ -1839,11 +1845,6 @@ EventStateManager::GenerateDragGesture(nsPresContext* aPresContext,
                                          nullptr);
       }
 
-      // now that the dataTransfer has been updated in the dragstart and
-      // draggesture events, make it read only so that the data doesn't
-      // change during the drag.
-      dataTransfer->SetMode(DataTransfer::Mode::ReadOnly);
-
       if (status != nsEventStatus_eConsumeNoDefault) {
         bool dragStarted = DoDefaultDragStart(aPresContext, event, dataTransfer,
                                               targetContent, selection);
@@ -2006,6 +2007,18 @@ EventStateManager::DoDefaultDragStart(nsPresContext* aPresContext,
   if (!transArray)
     return false;
 
+  // After this function returns, the DataTransfer will be cleared so it appears
+  // empty to content. We need to pass a DataTransfer into the Drag Session, so
+  // we need to make a copy.
+  RefPtr<DataTransfer> dataTransfer;
+  aDataTransfer->Clone(aDragTarget, eDrop, aDataTransfer->MozUserCancelled(),
+                       false, getter_AddRefs(dataTransfer));
+
+  // Copy over the drop effect, as Clone doesn't copy it for us.
+  uint32_t dropEffect;
+  aDataTransfer->GetDropEffectInt(&dropEffect);
+  dataTransfer->SetDropEffectInt(dropEffect);
+
   // XXXndeakin don't really want to create a new drag DOM event
   // here, but we need something to pass to the InvokeDragSession
   // methods.
@@ -2018,7 +2031,7 @@ EventStateManager::DoDefaultDragStart(nsPresContext* aPresContext,
   // other than a selection is being dragged.
   if (!dragImage && aSelection) {
     dragService->InvokeDragSessionWithSelection(aSelection, transArray,
-                                                action, event, aDataTransfer);
+                                                action, event, dataTransfer);
   }
   else {
     // if dragging within a XUL tree and no custom drag image was
@@ -2045,7 +2058,7 @@ EventStateManager::DoDefaultDragStart(nsPresContext* aPresContext,
                                             dragImage ? dragImage->AsDOMNode() :
                                                         nullptr,
                                             imageX, imageY, event,
-                                            aDataTransfer);
+                                            dataTransfer);
   }
 
   return true;
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index 5d4c117f21d8..9b1710cc4b58 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -8242,6 +8242,23 @@ PresShell::HandleEventInternal(WidgetEvent* aEvent,
     case eMouseMove:
       nsIPresShell::AllowMouseCapture(false);
       break;
+    case eDrag:
+    case eDragEnd:
+    case eDragEnter:
+    case eDragExit:
+    case eDragLeave:
+    case eDragOver:
+    case eDrop: {
+      // After any drag event other than dragstart (which is handled separately,
+      // as we need to collect the data first), the DataTransfer needs to be
+      // made protected, and then disconnected.
+      DataTransfer* dataTransfer = aEvent->AsDragEvent()->mDataTransfer;
+      if (dataTransfer) {
+        dataTransfer->SetMode(DataTransfer::Mode::Protected);
+        dataTransfer->ClearAll();
+      }
+      break;
+    }
     default:
       break;
     }