diff --git a/netwerk/system/mac/nsNetworkLinkService.h b/netwerk/system/mac/nsNetworkLinkService.h
index 57c0df963ffb..76a128b6757e 100644
--- a/netwerk/system/mac/nsNetworkLinkService.h
+++ b/netwerk/system/mac/nsNetworkLinkService.h
@@ -7,6 +7,7 @@
 
 #include "nsINetworkLinkService.h"
 #include "nsIObserver.h"
+#include "mozilla/Mutex.h"
 
 #include <SystemConfiguration/SCNetworkReachability.h>
 #include <SystemConfiguration/SystemConfiguration.h>
@@ -44,6 +45,8 @@ class nsNetworkLinkService : public nsINetworkLinkService, public nsIObserver {
   static void IPConfigChanged(SCDynamicStoreRef store, CFArrayRef changedKeys,
                               void* info);
   void calculateNetworkId(void);
+
+  mozilla::Mutex mMutex;
   nsCString mNetworkId;
 };
 
diff --git a/netwerk/system/mac/nsNetworkLinkService.mm b/netwerk/system/mac/nsNetworkLinkService.mm
index 221f340600ec..ebc1ecbebb00 100644
--- a/netwerk/system/mac/nsNetworkLinkService.mm
+++ b/netwerk/system/mac/nsNetworkLinkService.mm
@@ -26,6 +26,7 @@
 #include "mozilla/Base64.h"
 #include "mozilla/Telemetry.h"
 #include "nsNetworkLinkService.h"
+#include "MainThreadUtils.h"
 #include "../../base/IPv6Utils.h"
 
 #import <Cocoa/Cocoa.h>
@@ -74,7 +75,8 @@ nsNetworkLinkService::nsNetworkLinkService()
       mReachability(nullptr),
       mCFRunLoop(nullptr),
       mRunLoopSource(nullptr),
-      mStoreRef(nullptr) {}
+      mStoreRef(nullptr),
+      mMutex("nsNetworkLinkService::mMutex") {}
 
 nsNetworkLinkService::~nsNetworkLinkService() = default;
 
@@ -99,6 +101,13 @@ nsNetworkLinkService::GetLinkType(uint32_t* aLinkType) {
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsNetworkLinkService::GetNetworkID(nsACString& aNetworkID) {
+  MutexAutoLock lock(mMutex);
+  aNetworkID = mNetworkId;
+  return NS_OK;
+}
+
 #ifndef SA_SIZE
 #  define SA_SIZE(sa)                                 \
     ((!(sa) || ((struct sockaddr*)(sa))->sa_len == 0) \
@@ -315,6 +324,7 @@ static bool ipv6NetworkId(SHA1Sum* sha1) {
 }
 
 void nsNetworkLinkService::calculateNetworkId(void) {
+  MOZ_ASSERT(!NS_IsMainThread(), "Should not be called on the main thread");
   SHA1Sum sha1;
   bool found4 = ipv4NetworkId(&sha1);
   bool found6 = ipv6NetworkId(&sha1);
@@ -331,6 +341,7 @@ void nsNetworkLinkService::calculateNetworkId(void) {
     nsresult rv = Base64Encode(newString, output);
     MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv));
     LOG(("networkid: id %s\n", output.get()));
+    MutexAutoLock lock(mMutex);
     if (mNetworkId != output) {
       // new id
       if (found4 && !found6) {
@@ -343,10 +354,14 @@ void nsNetworkLinkService::calculateNetworkId(void) {
       mNetworkId = output;
     } else {
       // same id
+      LOG(("Same network id"));
       Telemetry::Accumulate(Telemetry::NETWORK_ID2, 2);
     }
   } else {
     // no id
+    LOG(("No network id"));
+    MutexAutoLock lock(mMutex);
+    mNetworkId.Truncate();
     Telemetry::Accumulate(Telemetry::NETWORK_ID2, 0);
   }
 }