--- a/xpcom/string/nsTextFormatter.cpp
+++ b/xpcom/string/nsTextFormatter.cpp
@@ -828,21 +828,18 @@ dosprintf(SprintfStateStr* aState, const
     int64_t ll;
     double d;
     const char* s;
     const char16_t* S;
     int* ip;
   } u;
   char16_t space = ' ';
 
-  nsAutoString hex;
-  hex.AssignLiteral("0123456789abcdef");
-
-  nsAutoString HEX;
-  HEX.AssignLiteral("0123456789ABCDEF");
+  static const char16_t hex[] = u"0123456789abcdef";
+  static char16_t HEX[] = u"0123456789ABCDEF";
 
   const char16_t* hexp;
   int rv, i;
   struct NumArgState* nas = nullptr;
   struct NumArgState  nasArray[NAS_DEFAULT_NUM];
 
 
   /*
@@ -976,17 +973,17 @@ dosprintf(SprintfStateStr* aState, const
       c = *aFmt++;
       if (c == 'l') {
         type = NumArgState::INT64;
         c = *aFmt++;
       }
     }
 
     /* format */
-    hexp = hex.get();
+    hexp = hex;
     switch (c) {
       case 'd':
       case 'i':                               /* decimal/integer */
         radix = 10;
         goto fetch_and_convert;
 
       case 'o':                               /* octal */
         radix = 8;
@@ -1000,17 +997,17 @@ dosprintf(SprintfStateStr* aState, const
 
       case 'x':                               /* unsigned hex */
         radix = 16;
         type |= 1;
         goto fetch_and_convert;
 
       case 'X':                               /* unsigned HEX */
         radix = 16;
-        hexp = HEX.get();
+        hexp = HEX;
         type |= 1;
         goto fetch_and_convert;
 
         fetch_and_convert:
         switch (type) {
           case NumArgState::INT16:
             u.l = va_arg(aAp, int);
             if (u.l < 0) {