diff --git a/js/public/MemoryMetrics.h b/js/public/MemoryMetrics.h
index 54faf160f40a..67566d6a31cf 100644
--- a/js/public/MemoryMetrics.h
+++ b/js/public/MemoryMetrics.h
@@ -123,6 +123,7 @@ struct ObjectsExtraSizes
     macro(Objects, NotLiveGCThing, mallocHeapElementsNonAsmJS) \
     macro(Objects, NotLiveGCThing, mallocHeapElementsAsmJS) \
     macro(Objects, NotLiveGCThing, nonHeapElementsAsmJS) \
+    macro(Objects, NotLiveGCThing, nonHeapElementsMapped) \
     macro(Objects, NotLiveGCThing, nonHeapCodeAsmJS) \
     macro(Objects, NotLiveGCThing, mallocHeapAsmJSModuleData) \
     macro(Objects, NotLiveGCThing, mallocHeapArgumentsData) \
diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index 24a5b2629c78..00ac52a1fb41 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -5908,6 +5908,8 @@ JSObject::addSizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf, JS::Objects
 #else
             sizes->mallocHeapElementsAsmJS += mallocSizeOf(elements);
 #endif
+        } else if (MOZ_UNLIKELY(elements->isMappedArrayBuffer())) {
+            sizes->nonHeapElementsMapped += as<ArrayBufferObject>().byteLength();
         } else {
             sizes->mallocHeapElementsNonAsmJS += mallocSizeOf(elements);
         }
diff --git a/js/xpconnect/src/XPCJSRuntime.cpp b/js/xpconnect/src/XPCJSRuntime.cpp
index 845c6bcade39..6b8c708eda69 100644
--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -2139,6 +2139,12 @@ ReportCompartmentStats(const JS::CompartmentStats &cStats,
             "the GC heap.");
     }
 
+    if (cStats.objectsExtra.nonHeapElementsMapped > 0) {
+        REPORT_BYTES(cJSPathPrefix + NS_LITERAL_CSTRING("objects/non-heap/elements/mapped"),
+            KIND_NONHEAP, cStats.objectsExtra.nonHeapElementsMapped,
+            "Memory-mapped array buffer elements.");
+    }
+
     REPORT_BYTES(cJSPathPrefix + NS_LITERAL_CSTRING("objects/non-heap/code/asm.js"),
         KIND_NONHEAP, cStats.objectsExtra.nonHeapCodeAsmJS,
         "AOT-compiled asm.js code.");