--- a/js/xpconnect/src/XPCLocale.cpp
+++ b/js/xpconnect/src/XPCLocale.cpp
@@ -12,16 +12,17 @@
 #include "nsJSUtils.h"
 #include "nsIPlatformCharset.h"
 #include "nsILocaleService.h"
 #include "nsICollation.h"
 #include "nsUnicharUtils.h"
 #include "nsComponentManagerUtils.h"
 #include "nsServiceManagerUtils.h"
 #include "mozilla/dom/EncodingUtils.h"
+#include "mozilla/Preferences.h"
 #include "nsIUnicodeDecoder.h"
 
 #include "xpcpublic.h"
 
 using namespace JS;
 using mozilla::dom::EncodingUtils;
 
 /**
@@ -247,16 +248,25 @@ private:
 };
 
 bool
 xpc_LocalizeRuntime(JSRuntime* rt)
 {
   JS_SetLocaleCallbacks(rt, new XPCLocaleCallbacks());
 
   // Set the default locale.
+
+  // Check a pref to see if we should use US English locale regardless
+  // of the system locale.
+  if (Preferences::GetBool("javascript.use_us_english_locale", false)) {
+    return JS_SetDefaultLocale(rt, "en-US");
+  }
+
+  // No pref has been found, so get the default locale from the
+  // application's locale.
   nsCOMPtr<nsILocaleService> localeService =
     do_GetService(NS_LOCALESERVICE_CONTRACTID);
   if (!localeService)
     return false;
 
   nsCOMPtr<nsILocale> appLocale;
   nsresult rv = localeService->GetApplicationLocale(getter_AddRefs(appLocale));
   if (NS_FAILED(rv))

--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -140,16 +140,17 @@
 #include "nsToolkitCompsCID.h"
 
 #include "nsINIParser.h"
 #include "mozilla/Omnijar.h"
 #include "mozilla/StartupTimeline.h"
 #include "mozilla/LateWriteChecks.h"
 
 #include <stdlib.h>
+#include <locale.h>
 
 #ifdef XP_UNIX
 #include <sys/stat.h>
 #include <unistd.h>
 #include <pwd.h>
 #endif
 
 #ifdef XP_WIN
@@ -4145,16 +4146,18 @@ XREMain::XRE_mainRun()
       // Set the new profile as the default after we're done cleaning up the old default.
       rv = SetCurrentProfileAsDefault(mProfileSvc, mProfD);
       if (NS_FAILED(rv)) NS_WARNING("Could not set current profile as the default");
     }
   }
 
   mDirProvider.DoStartup();
 
+  OverrideDefaultLocaleIfNeeded();
+
 #ifdef MOZ_CRASHREPORTER
   nsCString userAgentLocale;
   // Try a localized string first. This pref is always a localized string in
   // Fennec, and might be elsewhere, too.
   if (NS_SUCCEEDED(Preferences::GetLocalizedCString("general.useragent.locale", &userAgentLocale))) {
     CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("useragent_locale"), userAgentLocale);
   } else if (NS_SUCCEEDED(Preferences::GetCString("general.useragent.locale", &userAgentLocale))) {
     CrashReporter::AnnotateCrashReport(NS_LITERAL_CSTRING("useragent_locale"), userAgentLocale);
@@ -4803,8 +4806,20 @@ SetupErrorHandling(const char* progname)
   _CrtSetReportHook(MSCRTReportHook);
 #endif
 
   InstallSignalHandlers(progname);
 
   // Unbuffer stdout, needed for tinderbox tests.
   setbuf(stdout, 0);
 }
+
+void
+OverrideDefaultLocaleIfNeeded() {
+  // Read pref to decide whether to override default locale with US English.
+  if (mozilla::Preferences::GetBool("javascript.use_us_english_locale", false)) {
+    // Set the application-wide C-locale. Needed to resist fingerprinting
+    // of Date.toLocaleFormat(). We use the locale to "C.UTF-8" if possible,
+    // to avoid interfering with non-ASCII keyboard input on some Linux desktops.
+    // Otherwise fall back to the "C" locale, which is available on all platforms.
+    setlocale(LC_ALL, "C.UTF-8") || setlocale(LC_ALL, "C");
+  }
+}

--- a/toolkit/xre/nsAppRunner.h
+++ b/toolkit/xre/nsAppRunner.h
@@ -88,16 +88,19 @@ NS_NewToolkitProfileFactory(nsIFactory* 
  */
 nsresult
 NS_LockProfilePath(nsIFile* aPath, nsIFile* aTempPath,
                    nsIProfileUnlocker* *aUnlocker, nsIProfileLock* *aResult);
 
 void
 WriteConsoleLog();
 
+void
+OverrideDefaultLocaleIfNeeded();
+
 #ifdef XP_WIN
 void
 UseParentConsole();
 
 BOOL
 WinLaunchChild(const wchar_t *exePath, int argc,
                char **argv, HANDLE userToken = nullptr,
                HANDLE *hProcess = nullptr);

--- a/toolkit/xre/nsEmbedFunctions.cpp
+++ b/toolkit/xre/nsEmbedFunctions.cpp
@@ -614,16 +614,18 @@ XRE_InitChildProcess(int aArgc,
 #endif
 
 #if defined(MOZ_SANDBOX) && defined(XP_WIN)
       // We need to do this after the process has been initialised, as
       // InitLoggingIfRequired may need access to prefs.
       mozilla::sandboxing::InitLoggingIfRequired();
 #endif
 
+      OverrideDefaultLocaleIfNeeded();
+
       // Run the UI event loop on the main thread.
       uiMessageLoop.MessageLoop::Run();
 
       // Allow ProcessChild to clean up after itself before going out of
       // scope and being deleted
       process->CleanUp();
       mozilla::Omnijar::CleanUp();
     }