drupal.org gives "Warning: Cannot modify header information"

When I visit the drupal.org homepage as an anonymous user, I get the following errors at the top of page, behind the usual front page:

Warning: Cannot modify header information - headers already sent by (output started at /var/www/drupal.org/htdocs/includes/database.mysql.inc:172) in /var/www/drupal.org/htdocs/includes/bootstrap.inc on line 488

Warning: Cannot modify header information - headers already sent by (output started at /var/www/drupal.org/htdocs/includes/database.mysql.inc:172) in /var/www/drupal.org/htdocs/includes/bootstrap.inc on line 489

Warning: Cannot modify header information - headers already sent by (output started at /var/www/drupal.org/htdocs/includes/database.mysql.inc:172) in /var/www/drupal.org/htdocs/includes/bootstrap.inc on line 490

Warning: Cannot modify header information - headers already sent by (output started at /var/www/drupal.org/htdocs/includes/database.mysql.inc:172) in /var/www/drupal.org/htdocs/includes/bootstrap.inc on line 491

If I visit another page, or once I've logged in, these errors go away. However if I log out again, the messages return.

I'm using Firefox 2.0.0.3 using AdBlock Plus plugin, on Windows XP SP2.