Use PDFView.error to avoid issues with alert.

web/viewer.js

@@ -1058,7 +1058,7 @@ var PDFView = {
     if (!this.supportsPrinting) {
       var printMessage = mozL10n.get('printing_not_supported', null,
           'Warning: Printing is not fully supported by this browser.');
-      alert(printMessage);
+      this.error(printMessage);
       return;
     }
     var body = document.querySelector('body');