Firebug. Every developer from around 2006-2008 knows what I'm talking about. Debugging JS in IE6 was like trying to build a house blindfolded with both arms tied behind your back. Firebug is when JS went from just a web augmentation toy that could silently fail and your web page would still mostly function to becoming a critical function for a web page (many will see this as all a big mistake).
> Debugging JS in IE6 Visual Studio (even back when it was called Visual Interdev) supported first-class script debugging with Internet Explorer going as far back as Internet Explorer 4. Ditto cscript/wscript. It just wasn't popular (and few people knew about it) because most people doing front-end web-work _back then_ weren't using Visual Studio, and in general most people weren't doing back-end web-work in Visual S…
I’ll add that to the other valid reasons that you mention.