diff --git a/mozilla/extensions/transformiix/source/base/Double.cpp b/mozilla/extensions/transformiix/source/base/Double.cpp
index 0cb2102bed5..66bac4234f4 100644
--- a/mozilla/extensions/transformiix/source/base/Double.cpp
+++ b/mozilla/extensions/transformiix/source/base/Double.cpp
@@ -253,34 +253,61 @@ void Double::toString(double aValue, nsAString& aDest)
         return;
     }
 
-    int bufsize;
-    if (fabs(aValue) > 1)
-        bufsize = (int)log10(fabs(aValue)) + 30;
-    else
-        bufsize = 30;
-    
-    char* buf = new char[bufsize];
-    if (!buf) {
-        NS_ASSERTION(0, "out of memory");
-        return;
-    }
+    // Mantissa length is 17, so this is plenty
+    const int buflen = 20;
+    char buf[buflen];
 
     PRIntn intDigits, sign;
     char* endp;
-    PR_dtoa(aValue, 0, 0, &intDigits, &sign, &endp, buf, bufsize-1);
+    PR_dtoa(aValue, 0, 0, &intDigits, &sign, &endp, buf, buflen - 1);
 
-    if (sign)
-        aDest.Append(PRUnichar('-'));
-
-    int i;
-    for (i = 0; i < endp - buf; i++) {
-        if (i == intDigits)
-            aDest.Append(PRUnichar('.'));
-        aDest.Append(PRUnichar(buf[i]));
+    // compute length
+    PRInt32 length = endp - buf;
+    if (length > intDigits) {
+        // decimal point needed
+        ++length;
+        if (intDigits < 1) {
+            // leading zeros, -intDigits + 1
+            length += 1 - intDigits;
+        }
+    }
+    else {
+        // trailing zeros, total length given by intDigits
+        length = intDigits;
+    }
+    if (aValue < 0)
+        ++length;
+    PRUint32 oldlength = aDest.Length();
+    aDest.SetLength(oldlength + length); // grow the string
+    nsAString::iterator dest;
+    aDest.BeginWriting(dest).advance(PRInt32(oldlength));
+    if (aValue < 0) {
+        *dest = '-'; ++dest;
+    }
+    int i;
+    // leading zeros
+    if (intDigits < 1) {
+        *dest = '0'; ++dest;
+        *dest = '.'; ++dest;
+        for (i = 0; i > intDigits; --i) {
+            *dest = '0'; ++dest;
+        }
+    }
+    // mantissa
+    int firstlen = PR_MIN(intDigits, endp - buf);
+    for (i = 0; i < firstlen; i++) {
+        *dest = buf[i]; ++dest;
+    }
+    if (i < endp - buf) {
+        if (i > 0) {
+            *dest = '.'; ++dest;
+        }
+        for (; i < endp - buf; i++) {
+            *dest = buf[i]; ++dest;
+        }
+    }
+    // trailing zeros
+    for (; i < intDigits; i++) {
+        *dest = '0'; ++dest;
     }
-    
-    for (; i < intDigits; i++)
-        aDest.Append(PRUnichar('0'));
-
-    delete [] buf;
 }