Error message in Firefox, but not Chrome

When I go to the main client area on my site in Firefox, I now get this message:

Warning: Cannot modify header information - headers already sent by (output started at /home/booktour/public_html/_sites/book-tours.com/wp-content/themes/canvas/header.php:11) in /home/booktour/public_html/_sites/book-tours.com/wp-includes/pluggable.php on line 1178

But in Chrome it shows without a problem.