Firebug as Web Inspector for FileMaker on Windows
We got it to work here for some development:

Allows to open a console, do some inspection in DOM tree and CSS rules. It even allows you to select object in the html page with mouse to inspect. And can optionally run in a separate window.
But sadly it is long outdated, so may not know latest JavaScript tricks or have some bugs. And doesn't work with more than 1x resolution of screen as far as I see. But otherwise may be a help here to debug JavaScript with IE 11 within FileMaker's web viewer. For installation in FileMaker, you can run the JavaScript to initialize via WebView.RunJavaScript via our plugin.