--- a/js/src/vm/DateTime.cpp
+++ b/js/src/vm/DateTime.cpp
@@ -67,23 +67,16 @@ ComputeUTCTime(time_t t, struct tm* ptm)
  */
 static int32_t
 UTCToLocalStandardOffsetSeconds()
 {
     using js::SecondsPerDay;
     using js::SecondsPerHour;
     using js::SecondsPerMinute;
 
-#if defined(XP_WIN)
-    // Windows doesn't follow POSIX: updates to the TZ environment variable are
-    // not reflected immediately on that platform as they are on other systems
-    // without this call.
-    _tzset();
-#endif
-
     // Get the current time.
     time_t currentMaybeWithDST = time(nullptr);
     if (currentMaybeWithDST == time_t(-1))
         return 0;
 
     // Break down the current time into its (locally-valued, maybe with DST)
     // components.
     struct tm local;
@@ -175,23 +168,16 @@ js::DateTimeInfo::DateTimeInfo()
 }
 
 int64_t
 js::DateTimeInfo::computeDSTOffsetMilliseconds(int64_t utcSeconds)
 {
     MOZ_ASSERT(utcSeconds >= 0);
     MOZ_ASSERT(utcSeconds <= MaxUnixTimeT);
 
-#if defined(XP_WIN)
-    // Windows does not follow POSIX. Updates to the TZ environment variable
-    // are not reflected immediately on that platform as they are on UNIX
-    // systems without this call.
-    _tzset();
-#endif
-
     struct tm tm;
     if (!ComputeLocalTime(static_cast<time_t>(utcSeconds), &tm))
         return 0;
 
     int32_t dayoff = int32_t((utcSeconds + utcToLocalStandardOffsetSeconds) % SecondsPerDay);
     int32_t tmoff = tm.tm_sec + (tm.tm_min * SecondsPerMinute) + (tm.tm_hour * SecondsPerHour);
 
     int32_t diff = tmoff - dayoff;

--- a/toolkit/components/resistfingerprinting/nsRFPService.cpp
+++ b/toolkit/components/resistfingerprinting/nsRFPService.cpp
@@ -1,23 +1,26 @@
 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsRFPService.h"
 
+#include <time.h>
+
 #include "mozilla/ClearOnShutdown.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/Services.h"
 #include "mozilla/StaticPtr.h"
 
 #include "nsCOMPtr.h"
 #include "nsServiceManagerUtils.h"
 #include "nsString.h"
+#include "nsXULAppAPI.h"
 
 #include "nsIObserverService.h"
 #include "nsIPrefBranch.h"
 #include "nsIPrefService.h"
 #include "nsJSUtils.h"
 
 #include "prenv.h"
 
@@ -115,16 +118,29 @@ nsRFPService::Init()
   const char* tzValue = PR_GetEnv("TZ");
   if (tzValue) {
     mInitialTZValue = nsCString(tzValue);
   }
 
   // Call UpdatePref() here to cache the value of 'privacy.resistFingerprinting'
   // and set the timezone.
   UpdatePref();
+
+#if defined(XP_WIN)
+  // If we're e10s, then we don't need to run this, since the child process will
+  // simply inherit the environment variable from the parent process, in which
+  // case it's unnecessary to call _tzset().
+  if (XRE_IsParentProcess() && !XRE_IsE10sParentProcess()) {
+    // Windows does not follow POSIX. Updates to the TZ environment variable
+    // are not reflected immediately on that platform as they are on UNIX
+    // systems without this call.
+    _tzset();
+  }
+#endif
+
   return rv;
 }
 
 void
 nsRFPService::UpdatePref()
 {
   MOZ_ASSERT(NS_IsMainThread());
   sPrivacyResistFingerprinting = Preferences::GetBool(RESIST_FINGERPRINTING_PREF);
@@ -147,18 +163,16 @@ nsRFPService::UpdatePref()
 #else
       // For Windows, we reset the TZ to an empty string. This will make Windows to use
       // its system timezone.
       PR_SetEnv("TZ=");
 #endif
     }
   }
 
-  // We don't have to call _tzset() here for Windows since the following
-  // function nsJSUtils::ResetTimeZone() will call it for us.
   nsJSUtils::ResetTimeZone();
 }
 
 void
 nsRFPService::StartShutdown()
 {
   MOZ_ASSERT(NS_IsMainThread());
 
@@ -179,16 +193,25 @@ NS_IMETHODIMP
 nsRFPService::Observe(nsISupports* aObject, const char* aTopic,
                       const char16_t* aMessage)
 {
   if (!strcmp(NS_PREFBRANCH_PREFCHANGE_TOPIC_ID, aTopic)) {
     NS_ConvertUTF16toUTF8 pref(aMessage);
 
     if (pref.EqualsLiteral(RESIST_FINGERPRINTING_PREF)) {
       UpdatePref();
+
+#if defined(XP_WIN)
+      if (!XRE_IsE10sParentProcess()) {
+        // Windows does not follow POSIX. Updates to the TZ environment variable
+        // are not reflected immediately on that platform as they are on UNIX
+        // systems without this call.
+        _tzset();
+      }
+#endif
     }
   }
 
   if (!strcmp(NS_XPCOM_SHUTDOWN_OBSERVER_ID, aTopic)) {
     StartShutdown();
   }
 
   return NS_OK;