Don't use WP_PLUGIN_URL

Please change this:
$font_resizer_path =WP_PLUGIN_URL.'/font-resizer/js/';
to
$font_resizer_path = plugins_url( 'js/', __FILE__ );

This will make your plugin usable in SSL sites.

thanks!

https://ww.wp.xz.cn/plugins/font-resizer/