I'm glad to see Firebug being replace with a more modern set of tools, but you're right - Firebug (and the webkit devtools that followed from it) is/was amazing. It was probably the most significant change in front-end web development I've experienced in my 15ish year career.
XMLHttpRequest was released by MS in IE 5 in 1999, which is less than 15 years ago. I'd say that the development of ajax was far more significant than some dev tools.
And as a slight nitpick, the IE Dev toolbar came out about 6 months before Firebug and as best I can remember actually had more functionality than Firebug for quite a while.
It's some sort of persistent web developer myth that Firebug came first. Firebug was just easier to find/install, you had to know that the IE6 developer toolbar existed.
NB: I hate IE as much as the next web dev, but credit where credit's due...
