Insecure content warning with D7 and SSL
Simply remove $base_url from your settings.php file; Drupal will detect $base_url automatically.

If you must set this value explicitly, you can add a little conditional logic to your settings file, such as:

if (isset($_SERVER['HTTPS']) &amp;&amp; strtolower($_SERVER['HTTPS']) == 'on') {
  $base_url = 'https://example.com';
}
else {
  $base_url = 'http://example.com';
}