http://console2.mozdev.org/

Console² , is used not only for JavaScript errors, but for CSS errors as well. This extension takes this into account by providing a simple possibility to display errors by type. It let's you display errors filtered by type (Errors, Warnings, Messages), language (JavaScript, CSS, XML) and context (Chrome, Content). Furthermore it provides a simple search box (as seen in the History and Bookmarks sidebars) and some accessibility improvements.