Chances are I’m way behind on blogging this, but this looks like a nice replacement for the default Firefox error console. Not to say Firefox doesn’t do everything right the first time…*cough, cough*,,, Download the frk’n extension! 
Console² 0.2.5 [Nov 02] – MozillaZine Forums
Console² 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.