diff --git a/netwerk/dns/PlatformDNSUnix.cpp b/netwerk/dns/PlatformDNSUnix.cpp
index 0acf4d024a6..c104146a42e 100644
--- a/netwerk/dns/PlatformDNSUnix.cpp
+++ b/netwerk/dns/PlatformDNSUnix.cpp
@@ -9,6 +9,7 @@
 #include "mozilla/net/DNSPacket.h"
 #include "nsIDNSService.h"
 #include "mozilla/Maybe.h"
+#include "mozilla/Mutex.h"
 #include "mozilla/StaticPrefs_network.h"
 #include "mozilla/ThreadLocal.h"
 
@@ -22,6 +23,7 @@ namespace mozilla::net {
 
 #if defined(HAVE_RES_NINIT)
 MOZ_THREAD_LOCAL(struct __res_state*) sThreadRes;
+mozilla::StaticMutex sMutex MOZ_UNANNOTATED;
 #endif
 
 #define LOG(msg, ...) \
@@ -44,9 +46,12 @@ nsresult ResolveHTTPSRecordImpl(const nsACString& aHost,
   if (!sThreadRes.get()) {
     UniquePtr<struct __res_state> resState(new struct __res_state);
     memset(resState.get(), 0, sizeof(struct __res_state));
-    if (int ret = res_ninit(resState.get())) {
-      LOG("res_ninit failed: %d", ret);
-      return NS_ERROR_UNKNOWN_HOST;
+    {
+      StaticMutexAutoLock lock(sMutex);
+      if (int ret = res_ninit(resState.get())) {
+        LOG("res_ninit failed: %d", ret);
+        return NS_ERROR_UNKNOWN_HOST;
+      }
     }
     sThreadRes.set(resState.release());
   }
@@ -90,7 +95,10 @@ void DNSThreadShutdown() {
   }
 
   sThreadRes.set(nullptr);
-  res_nclose(res);
+  {
+    StaticMutexAutoLock lock(sMutex);
+    res_nclose(res);
+  }
   free(res);
 #endif
 }