--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -10,16 +10,17 @@
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/ChaosMode.h"
 #include "mozilla/IOInterposer.h"
 #include "mozilla/Likely.h"
 #include "mozilla/MemoryChecking.h"
 #include "mozilla/Poison.h"
 #include "mozilla/Preferences.h"
+#include "mozilla/Printf.h"
 #include "mozilla/ScopeExit.h"
 #include "mozilla/Services.h"
 #include "mozilla/Telemetry.h"
 
 #include "nsAppRunner.h"
 #include "mozilla/XREAppData.h"
 #include "mozilla/Bootstrap.h"
 #if defined(MOZ_UPDATER) && !defined(MOZ_WIDGET_ANDROID)
@@ -419,23 +420,23 @@ static void UnexpectedExit() {
  * Output a string to the user.  This method is really only meant to be used to
  * output last-ditch error messages designed for developers NOT END USERS.
  *
  * @param isError
  *        Pass true to indicate severe errors.
  * @param fmt
  *        printf-style format string followed by arguments.
  */
-static void Output(bool isError, const char *fmt, ... )
+static MOZ_FORMAT_PRINTF(2, 3) void Output(bool isError, const char *fmt, ... )
 {
   va_list ap;
   va_start(ap, fmt);
 
 #if defined(XP_WIN) && !MOZ_WINCONSOLE
-  char *msg = PR_vsmprintf(fmt, ap);
+  char *msg = mozilla::Vsmprintf(fmt, ap);
   if (msg)
   {
     UINT flags = MB_OK;
     if (isError)
       flags |= MB_ICONERROR;
     else
       flags |= MB_ICONINFORMATION;
 
@@ -443,17 +444,17 @@ static void Output(bool isError, const c
     MultiByteToWideChar(CP_ACP,
                         0,
                         msg,
                         -1,
                         wide_msg,
                         sizeof(wide_msg) / sizeof(wchar_t));
 
     MessageBoxW(nullptr, wide_msg, L"XULRunner", flags);
-    PR_smprintf_free(msg);
+    mozilla::SmprintfFree(msg);
   }
 #else
   vfprintf(stderr, fmt, ap);
 #endif
 
   va_end(ap);
 }
 
@@ -4339,19 +4340,19 @@ XREMain::XRE_mainRun()
   }
 
 #ifdef XP_WIN
   // Hack to sync up the various environment storages. XUL_APP_FILE is special
   // in that it comes from a different CRT (firefox.exe's static-linked copy).
   // Ugly details in http://bugzil.la/1175039#c27
   char appFile[MAX_PATH];
   if (GetEnvironmentVariableA("XUL_APP_FILE", appFile, sizeof(appFile))) {
-    char* saved = PR_smprintf("XUL_APP_FILE=%s", appFile);
+    char* saved = mozilla::Smprintf("XUL_APP_FILE=%s", appFile);
     PR_SetEnv(saved);
-    PR_smprintf_free(saved);
+    mozilla::SmprintfFree(saved);
   }
 #endif
 
   SaveStateForAppInitiatedRestart();
 
   // clear out any environment variables which may have been set
   // during the relaunch process now that we know we won't be relaunching.
   SaveToEnv("XRE_PROFILE_PATH=");