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).
Firebug came out AFTER IEDevToolbar, and IEDevToolbar was superior to Firebug for years for debugging JavaScript. So not a very good example at all, in fact quite the opposite, it just happened to ride the back of the anti-M$ wave and wasn't at all innovative, copying its features from already existing products. Conversely, though it was free, IEDevToolbar was only really advertised to MSDN subscribers, so never got…
It's not who was first, just who seemed 10x better than the next best option.