The JavaScript error console is a relic from the Mozilla stone age. There are two problems with it:

1. It lives in a separate window
2. It is not helpful I’ve been drooling over the new Firefox Web Console, but unfortunately it doesn’t have chrome privs [yet?] so it isn’t a replacement for the error console since it operates with privileges of the webpage displayed. Turns out I was 2 steps away from nirvana:
3. Navigate to chrome://global/content/console.xul.
4. Menu -> “Web Developer” -> “Web Console” Turns out that one can load a url with chrome privs (about:memory works too) and the web console becomes a very helpful [with autocompletion, less noise, etc) tool for typing Components.interfaces…

Update:

Scratchpad can also do this sans autocompletion, see Rob’s comment below.