diff --git a/image/imgRequest.cpp b/image/imgRequest.cpp
index ba99779d30..2a8af98cf4 100644
--- a/image/imgRequest.cpp
+++ b/image/imgRequest.cpp
@@ -96,6 +96,8 @@ imgRequest::Init(nsIURI *aURI,
                  ReferrerPolicy aReferrerPolicy)
 {
   MOZ_ASSERT(NS_IsMainThread(), "Cannot use nsIURI off main thread!");
+  // Init() can only be called once, and that's before it can be used off
+  // mainthread
 
   LOG_FUNC(gImgLog, "imgRequest::Init");
 
@@ -800,7 +802,14 @@ imgRequest::OnStopRequest(nsIRequest* aRequest,
 
   RefPtr<imgRequest> strongThis = this;
 
-  if (mIsMultiPartChannel && mNewPartPending) {
+  bool isMultipart = false;
+  bool newPartPending = false;
+  {
+    MutexAutoLock lock(mMutex);
+    isMultipart = mIsMultiPartChannel;
+    newPartPending = mNewPartPending;
+  }
+  if (isMultipart && newPartPending) {
     OnDataAvailable(aRequest, ctxt, nullptr, 0, 0);
   }