FirePHP - Firefox Extension to log PHP output

FirePHP is a Firefox extension (FireBug required) enables you to log to your Firebug console. using simple PHP method call. All data is sent via response headers and will not interfere with the content on your page. FirePHP is ideally suited for AJAX development where clean JSON and XML response are required.

<?php
FB::log('Log message');
FB::info('Info message');
FB::warn('Warn message');
FB::error('Error message');
?>

More detailed user guide.