Yes - it seems strange that this has not already been integrated into Google DevTools which I generally use. Perhaps there is a similar Chrome extension or Firebug add-in but have not see it.

Not the same thing but there is already a jslint Chrome extension <1> and a proposal to add jshint to DevTools itself <2>.

<1> https://chrome.google.com/webstore/detail/jslint-for-google-...

<2> https://groups.google.com/forum/#!topic/google-chrome-develo...

Safari Web inspector is open source now, someone can integrate and submit a patch for it as well. I prefer Safari over Chrome.

That would certainly be cool. But part of the value of this tool is its ability to customize it yourself.