Syntax error:  (line 1) in Firebug
The reason was a failed loaded javascript file.
I use the following code to add a JavaScript file:

drupal_add_js($p . '/slider.js','module','footer');


So the following HTML was added in the footer of the page:

&lt;script type="text/javascript" src="/sites/default/modules/custom/slider/slider.js?G"&gt;&lt;/script&gt;


At some point I renamed the file slider.js, so it stopped being loaded, and this is the reason behind the Firebug error.

In the "Net" tab, Firebug was not showing any error; it was showing just a "301 Moved Permanently" next to the slider.js file.