diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index 26398e1d4ca8..762b35d20122 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -897,7 +897,7 @@ PresShell::~PresShell() {
                    mLastCallbackEventRequest == nullptr,
                "post-reflow queues not empty.  This means we're leaking");
 
-  MOZ_ASSERT(mAllocatedPointers->IsEmpty(),
+  MOZ_ASSERT(!mAllocatedPointers || mAllocatedPointers->IsEmpty(),
              "Some pres arena objects were not freed");
 
   mFrameManager = nullptr;
diff --git a/layout/base/PresShell.h b/layout/base/PresShell.h
index bfba01084be2..9704b01b3e6f 100644
--- a/layout/base/PresShell.h
+++ b/layout/base/PresShell.h
@@ -1755,13 +1755,25 @@ class PresShell final : public nsStubDocumentObserver,
 
   void RecordAlloc(void* aPtr) {
 #ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    if (!mAllocatedPointers) {
+      return;  // Hash set was presumably freed to avert OOM.
+    }
     MOZ_DIAGNOSTIC_ASSERT(!mAllocatedPointers->Contains(aPtr));
-    mAllocatedPointers->Insert(aPtr);
+    if (!mAllocatedPointers->Insert(aPtr, fallible)) {
+      // Yikes! We're nearly out of memory, and this insertion would've pushed
+      // us over the ledge. At this point, we discard & stop using this set,
+      // since we don't have enough memory to keep it accurate from this point
+      // onwards. Hopefully this helps relieve the memory pressure a bit, too.
+      mAllocatedPointers = nullptr;
+    }
 #endif
   }
 
   void RecordFree(void* aPtr) {
 #ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    if (!mAllocatedPointers) {
+      return;  // Hash set was presumably freed to avert OOM.
+    }
     MOZ_DIAGNOSTIC_ASSERT(mAllocatedPointers->Contains(aPtr));
     mAllocatedPointers->Remove(aPtr);
 #endif
@@ -2858,8 +2870,13 @@ class PresShell final : public nsStubDocumentObserver,
   nsCOMPtr<nsITimer> mReflowContinueTimer;
 
 #ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
-  // We track allocated pointers in a debug-only hashtable to assert against
-  // missing/double frees.
+  // We track allocated pointers in a diagnostic hash set, to assert against
+  // missing/double frees. This set is allocated infallibly in the PresShell
+  // constructor's initialization list. The set can get quite large, so we use
+  // fallible allocation when inserting into it; and if these operations ever
+  // fail, then we just get rid of the set and stop using this diagnostic from
+  // that point on.  (There's not much else we can do, when the set grows
+  // larger than the available memory.)
   UniquePtr<nsTHashSet<void*>> mAllocatedPointers;
 #endif