Show Javascript error count in Firefox without opening console

Question

Now that Firebug has been subsumed into Firefox, how can I configure the browser to bring Javascript errors to my attention without having the console open all the time, as Firebug used to do by showing an error count on its toolbar icon?

Answer 1

The Firefox DevTools provide a so-called "Developer Toolbar" aka GCLI (openable via Firefox menu > Developer > Developer Toolbar or Shift + F2), which displays the number of errors on the page (besides providing many useful commands):

Update:

The DevTools team obviously plans to remove the Developer Toolbar, because of low usage and because it's unmaintained and for most of it's features are available somewhere else.

There is no replacement yet for the error count, though, but a comment on the related bug report indicates that it will be added back.