When I develop themes I always have debug turned on and these two notices cropped up when I activated font-resizer. Any chance you can update the plugin based on the info below would be greatly appreciated by all I'm sure.
Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in C:\xampp\htdocs\example\wp-includes\functions.php on line 2839
Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in C:\xampp\htdocs\example\wp-includes\functions.php on line 2923
All the best, Karl