--- a/xpcom/string/nsTextFormatter.cpp
+++ b/xpcom/string/nsTextFormatter.cpp
@@ -1198,41 +1198,38 @@ StringStuff(SprintfStateStr* aState, con
   str->Append(aStr, aLen);
 
   aState->base = str->BeginWriting();
   aState->cur = aState->base + off;
 
   return 0;
 }
 
-uint32_t
+void
 nsTextFormatter::ssprintf(nsAString& aOut, const char16_t* aFmt, ...)
 {
   va_list ap;
-  uint32_t rv;
 
   va_start(ap, aFmt);
-  rv = nsTextFormatter::vssprintf(aOut, aFmt, ap);
+  nsTextFormatter::vssprintf(aOut, aFmt, ap);
   va_end(ap);
-  return rv;
 }
 
-uint32_t
+void
 nsTextFormatter::vssprintf(nsAString& aOut, const char16_t* aFmt, va_list aAp)
 {
   SprintfStateStr ss;
   ss.stuff = StringStuff;
   ss.base = 0;
   ss.cur = 0;
   ss.maxlen = 0;
   ss.stuffclosure = &aOut;
 
   aOut.Truncate();
-  int n = dosprintf(&ss, aFmt, aAp);
-  return n ? n - 1 : n;
+  dosprintf(&ss, aFmt, aAp);
 }
 
 /*
 ** Stuff routine that discards overflow data
 */
 static int
 LimitStuff(SprintfStateStr* aState, const char16_t* aStr, uint32_t aLen)
 {
@@ -1281,19 +1278,23 @@ nsTextFormatter::vsnprintf(char16_t* aOu
   if ((int32_t)aOutLen <= 0) {
     return 0;
   }
 
   ss.stuff = LimitStuff;
   ss.base = aOut;
   ss.cur = aOut;
   ss.maxlen = aOutLen;
-  (void) dosprintf(&ss, aFmt, aAp);
+  int result = dosprintf(&ss, aFmt, aAp);
 
   /* If we added chars, and we didn't append a null, do it now. */
   if ((ss.cur != ss.base) && (*(ss.cur - 1) != '\0')) {
     *(--ss.cur) = '\0';
   }
 
+  if (result < 0) {
+    return -1;
+  }
+
   n = ss.cur - ss.base;
   return n ? n - 1 : n;
 }
 

--- a/xpcom/string/nsTextFormatter.h
+++ b/xpcom/string/nsTextFormatter.h
@@ -50,21 +50,21 @@ public:
    */
   static uint32_t snprintf(char16_t* aOut, uint32_t aOutLen,
                            const char16_t* aFmt, ...);
 
   /*
    * sprintf into an existing nsAString, overwriting any contents it already
    * has. Infallible.
    */
-  static uint32_t ssprintf(nsAString& aOut, const char16_t* aFmt, ...);
+  static void ssprintf(nsAString& aOut, const char16_t* aFmt, ...);
 
 private:
 
   /*
    * va_list forms of the above.
    */
   static uint32_t vsnprintf(char16_t* aOut, uint32_t aOutLen, const char16_t* aFmt,
                             va_list aAp);
-  static uint32_t vssprintf(nsAString& aOut, const char16_t* aFmt, va_list aAp);
+  static void vssprintf(nsAString& aOut, const char16_t* aFmt, va_list aAp);
 };
 
 #endif /* nsTextFormatter_h___ */