center the certificate manager by default. Remove the width attribute, let the dialog size itself intrinsically.

mail/extensions/smime/content/am-smime.js

@@ -407,5 +407,5 @@ function openCertManager()
     lastCertManager.focus();
   else
     window.open('chrome://pippki/content/certManager.xul',  "",
-                'chrome,width=500,height=400,resizable=yes,dialog=no');
+                'chrome,height=400,centerscreen,resizable=yes,dialog=no');
 }