MDC has documentation on how developers can log to the Error Console: https://developer.mozilla.org/en/error_console To replace the Error Console with the Web Console, the functions listed on this page will need to be backed by the new console service.

We already consume and surface some of the calls to Cu.reportError. The biggest issue here is handling toJavascriptConsole() calls. To which console do we log when the call comes from a toolbar or menubar-only extension? What if there is no browser window open (a common case on Mac OS). Sounds like we need to make the console work in its own window, stat. Perhaps the best way to handle the UI for this if there a console open already is to add a new filter checkbox that is off by default for "Browser Messages" once clicked, we interleave the XPConnect and chrome errors to one or more consoles

are we really doing this?

We're not doing this right now, certainly. Ultimately, though, I think we want to replace the Error Console with the Web Console.

We want to replace the Error Console with a 'global app console' that has the new features of the web console. Perhaps this bug can morph in that direction or we can create a new one for this?

I'd be ok with morphing that way. If we're duplicating/Replacing the Error Console watching the nsIConsoleService and friends should get us the compatibility we need.

Doesn't sound like this particular bug is needed, given that bug 587757 is going to give us a functionally equivalent console.