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).
I worked at Amazon during this time on our custom web server and before that I had been using telnet to hand type my commands or various perl scripts I wrote to emulate a browser's requests. And... ugh, tcpdump to figure out what exactly browsers were sending before they got to apache.
Firebug was like magic!