Possibly useful to anyone testing a site in Firefox, I just found out you can view the error console in a tab by going to the URL:
chrome://browser/content/places/places.xul

Also of interest:

Library (bookmarks):
chrome://browser/content/places/places.xul

Downloads:
chrome://mozapps/content/downloads/downloads.xul

Add-ons:
chrome://mozapps/content/extensions/extensions.xul

You’ll need to copy and paste these into your address bar as links don’t work, also they only work in Firefox (and possibly other Mozilla browsers).

[From: MozillaZine KB]