diff --git a/js/src/js.msg b/js/src/js.msg
index f21a371eff..5125cb12b8 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -445,6 +445,7 @@ MSG_DEF(JSMSG_SC_UNSUPPORTED_TYPE,     0, JSEXN_TYPEERR, "unsupported type for s
 MSG_DEF(JSMSG_SC_NOT_CLONABLE,         1, JSEXN_TYPEERR, "{0} cannot be cloned in this context")
 MSG_DEF(JSMSG_SC_SAB_TRANSFER,         0, JSEXN_WARN, "SharedArrayBuffer must not be in the transfer list")
 MSG_DEF(JSMSG_SC_SAB_DISABLED,         0, JSEXN_TYPEERR, "SharedArrayBuffer not cloned - shared memory disabled in receiver")
+MSG_DEF(JSMSG_SC_SAB_TOO_MANY_REFS,    0, JSEXN_TYPEERR, "SharedArrayBuffer has too many references")
 
 // Debugger
 MSG_DEF(JSMSG_ASSIGN_FUNCTION_OR_NULL, 1, JSEXN_TYPEERR, "value assigned to {0} must be a function or null")
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 387c78849f..6dd903e20b 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -5347,25 +5347,31 @@ GetSharedArrayBuffer(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
     JSObject* newObj = nullptr;
-    bool rval = true;
 
-    sharedArrayBufferMailboxLock->lock();
-    SharedArrayRawBuffer* buf = sharedArrayBufferMailbox;
-    if (buf) {
-        buf->addReference();
-        // Shared memory is enabled globally in the shell: there can't be a worker
-        // that does not enable it if the main thread has it.
-        MOZ_ASSERT(cx->compartment()->creationOptions().getSharedMemoryAndAtomicsEnabled());
-        newObj = SharedArrayBufferObject::New(cx, buf);
-        if (!newObj) {
-            buf->dropReference();
-            rval = false;
+    {
+        sharedArrayBufferMailboxLock->lock();
+        auto unlockMailbox = MakeScopeExit([]() { sharedArrayBufferMailboxLock->unlock(); });
+
+        SharedArrayRawBuffer* buf = sharedArrayBufferMailbox;
+        if (buf) {
+            if (!buf->addReference()) {
+                JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_SC_SAB_REFCNT_OFLO);
+                return false;
+            }
+
+            // Shared memory is enabled globally in the shell: there can't be a worker
+            // that does not enable it if the main thread has it.
+            MOZ_ASSERT(cx->compartment()->creationOptions().getSharedMemoryAndAtomicsEnabled());
+            newObj = SharedArrayBufferObject::New(cx, buf);
+            if (!newObj) {
+                buf->dropReference();
+                return false;
+            }
         }
     }
-    sharedArrayBufferMailboxLock->unlock();
 
     args.rval().setObjectOrNull(newObj);
-    return rval;
+    return true;
 }
 
 static bool
@@ -5379,18 +5385,24 @@ SetSharedArrayBuffer(JSContext* cx, unsigned argc, Value* vp)
     }
     else if (args.get(0).isObject() && args[0].toObject().is<SharedArrayBufferObject>()) {
         newBuffer = args[0].toObject().as<SharedArrayBufferObject>().rawBufferObject();
-        newBuffer->addReference();
+        if (!newBuffer->addReference()) {
+            JS_ReportErrorASCII(cx, "Reference count overflow on SharedArrayBuffer");
+            return false;
+        }
     } else {
         JS_ReportErrorASCII(cx, "Only a SharedArrayBuffer can be installed in the global mailbox");
         return false;
     }
 
-    sharedArrayBufferMailboxLock->lock();
-    SharedArrayRawBuffer* oldBuffer = sharedArrayBufferMailbox;
-    if (oldBuffer)
-        oldBuffer->dropReference();
-    sharedArrayBufferMailbox = newBuffer;
-    sharedArrayBufferMailboxLock->unlock();
+    {
+        sharedArrayBufferMailboxLock->lock();
+        auto unlockMailbox = MakeScopeExit([]() { sharedArrayBufferMailboxLock->unlock(); });
+
+        SharedArrayRawBuffer* oldBuffer = sharedArrayBufferMailbox;
+        if (oldBuffer)
+            oldBuffer->dropReference();
+        sharedArrayBufferMailbox = newBuffer;
+    }
 
     args.rval().setUndefined();
     return true;
diff --git a/js/src/vm/SharedArrayObject.cpp b/js/src/vm/SharedArrayObject.cpp
index 44fe3b790d..6a3c6a91c3 100644
--- a/js/src/vm/SharedArrayObject.cpp
+++ b/js/src/vm/SharedArrayObject.cpp
@@ -166,16 +166,30 @@ SharedArrayRawBuffer::New(JSContext* cx, uint32_t length)
     return rawbuf;
 }
 
-void
+bool
 SharedArrayRawBuffer::addReference()
 {
     MOZ_ASSERT(this->refcount_ > 0);
-    ++this->refcount_; // Atomic.
+
+    // Be careful never to overflow the refcount field.
+    for (;;) {
+        uint32_t old_refcount = this->refcount_;
+        uint32_t new_refcount = old_refcount+1;
+        if (new_refcount == 0)
+            return false;
+        if (this->refcount_.compareExchange(old_refcount, new_refcount))
+            return true;
+    }
 }
 
 void
 SharedArrayRawBuffer::dropReference()
 {
+    // Normally if the refcount is zero then the memory will have been unmapped
+    // and this test may just crash, but if the memory has been retained for any
+    // reason we will catch the underflow here.
+    MOZ_RELEASE_ASSERT(this->refcount_ > 0);
+
     // Drop the reference to the buffer.
     uint32_t refcount = --this->refcount_; // Atomic.
     if (refcount)
diff --git a/js/src/vm/SharedArrayObject.h b/js/src/vm/SharedArrayObject.h
index 19048336cb..d6b18b61a3 100644
--- a/js/src/vm/SharedArrayObject.h
+++ b/js/src/vm/SharedArrayObject.h
@@ -91,7 +91,7 @@ class SharedArrayRawBuffer
 
     uint32_t refcount() const { return refcount_; }
 
-    void addReference();
+    [[nodiscard]] bool addReference();
     void dropReference();
 };
 
diff --git a/js/src/vm/StructuredClone.cpp b/js/src/vm/StructuredClone.cpp
index 79f2d23d96..e1f1b2777e 100644
--- a/js/src/vm/StructuredClone.cpp
+++ b/js/src/vm/StructuredClone.cpp
@@ -1201,9 +1201,12 @@ JSStructuredCloneWriter::writeSharedArrayBuffer(HandleObject obj)
     Rooted<SharedArrayBufferObject*> sharedArrayBuffer(context(), &CheckedUnwrap(obj)->as<SharedArrayBufferObject>());
     SharedArrayRawBuffer* rawbuf = sharedArrayBuffer->rawBufferObject();
 
-    // Avoids a race condition where the parent thread frees the buffer
+    // Adding the reference here avoids a race condition where the parent thread frees the buffer
     // before the child has accepted the transferable.
-    rawbuf->addReference();
+    if (!rawbuf->addReference()) {
+        JS_ReportErrorNumberASCII(context(), GetErrorMessage, nullptr, JSMSG_SC_SAB_TOO_MANY_REFS);
+        return false;
+    }
 
     intptr_t p = reinterpret_cast<intptr_t>(rawbuf);
     return out.writePair(SCTAG_SHARED_ARRAY_BUFFER_OBJECT, static_cast<uint32_t>(sizeof(p))) &&