--- a/layout/style/SharedStyleSheetCache.cpp
+++ b/layout/style/SharedStyleSheetCache.cpp
@@ -499,22 +499,24 @@ void SharedStyleSheetCache::CancelDeferr
 }
 
 void SharedStyleSheetCache::CancelLoadsForLoader(css::Loader& aLoader) {
   CancelDeferredLoadsForLoader(aLoader);
 
   // We can't stop in-progress loads because some other loader may care about
   // them.
   for (auto iter = mLoadingDatas.Iter(); !iter.Done(); iter.Next()) {
-    SheetLoadData* data = iter.Data();
-    do {
+    MOZ_DIAGNOSTIC_ASSERT(iter.Data(),
+                          "We weren't properly notified and the load was "
+                          "incorrectly dropped on the floor");
+    for (SheetLoadData* data = iter.Data(); data; data = data->mNext) {
       if (data->mLoader == &aLoader) {
         data->mIsCancelled = true;
       }
-    } while ((data = data->mNext));
+    }
   }
 }
 
 void SharedStyleSheetCache::RegisterLoader(css::Loader& aLoader) {
   MOZ_ASSERT(aLoader.GetDocument());
   mLoaderPrincipalRefCnt.LookupForAdd(aLoader.GetDocument()->NodePrincipal())
       .OrInsert([] { return 0; }) += 1;
 }

--- a/layout/style/SharedStyleSheetCache.h
+++ b/layout/style/SharedStyleSheetCache.h
@@ -119,21 +119,22 @@ class SharedStyleSheetCache final : publ
 
     bool Expired() const;
   };
 
   void WillStartPendingLoad(css::SheetLoadData&);
 
   nsDataHashtable<SheetLoadDataHashKey, CompleteSheet> mCompleteSheets;
   nsRefPtrHashtable<SheetLoadDataHashKey, css::SheetLoadData> mPendingDatas;
-  // The SheetLoadData pointers in mLoadingDatas below are weak references.
+  // The SheetLoadData pointers in mLoadingDatas below are weak references that
+  // get cleaned up when StreamLoader::OnStopRequest gets called.
   //
   // Note that we hold on to all sheet loads, even if in the end they happen not
   // to be cacheable.
-  nsDataHashtable<SheetLoadDataHashKey, css::SheetLoadData*> mLoadingDatas;
+  nsDataHashtable<SheetLoadDataHashKey, WeakPtr<css::SheetLoadData>> mLoadingDatas;
 
   // An origin-to-number-of-registered-documents count, in order to manage cache
   // eviction as described in RegisterLoader / UnregisterLoader.
   nsDataHashtable<PrincipalHashKey, uint32_t> mLoaderPrincipalRefCnt;
 
   static SharedStyleSheetCache* sInstance;
 };
 

--- a/layout/style/SheetLoadData.h
+++ b/layout/style/SheetLoadData.h
@@ -34,16 +34,18 @@ namespace css {
  *********************************************/
 
 static_assert(eAuthorSheetFeatures == 0 && eUserSheetFeatures == 1 &&
                   eAgentSheetFeatures == 2,
               "sheet parsing mode constants won't fit "
               "in SheetLoadData::mParsingMode");
 
 class SheetLoadData final : public PreloaderBase,
+                            // FIXME(bug 1653011): This is a bit unfortunate.
+                            public SupportsWeakPtr<SheetLoadData>,
                             public nsIRunnable,
                             public nsIThreadObserver {
   using MediaMatched = dom::LinkStyle::MediaMatched;
   using IsAlternate = dom::LinkStyle::IsAlternate;
   using IsPreload = Loader::IsPreload;
   using UseSystemPrincipal = Loader::UseSystemPrincipal;
 
  protected: