Changing the User Title
The page title of the user profile page (like example.com/user/1) is generated by format_username(). If you want to control the username everywhere it occurs in the site, you can implement hook_username_alter in a custom module.

If you just want to change title on the user profile page, you could add THEMENAME_preprocess_page to your template.php to override $variables['title']. An example (untested code):

&lt;?php
/**
 * Preprocess function for page.tpl.php.
 * Replace 'THEMENAME' with the name of your theme.
 */
function THEMENAME_preprocess_page(&amp;$variables) {
  // Only act when the first part of the url equals 'user', the second
  // part is a number and the third part is empty.
  $arg = arg();
  if ($arg[0] == 'user' &amp;&amp; is_numeric($arg[1]) &amp;&amp; empty($arg[2])) {
    // Do stuff to create the desired username and set the $title variable for the template.
    $variables['title'] = $variables['firstname'] . ' ' . $variables['lastname'];
  }
}
?&gt;