XF 1.5 Access Developer Tools?

An easy solution is to add such a line to the config.php.

if (isset($_COOKIE['debug']))
    if ($_COOKIE['debug'] == 'J2WpYsqAKM991kdI') { $config['debug'] = true; }
Replace J2WpYsqAKM991kdI with your own secret key. Then create a new bookmark on your browser and enter the following code to the URL:

javascript:(function() {document.cookie='debug='+'J2WpYsqAKM991kdI'+';path=/;';})()
Again, don't forget to replace the secret key. Whenever you want to enable debug mode, just go to this bookmark when you're on your site (not on a blank page). Since it's a session cookie, the debug mode will be disabled when you close your browser windows.