That's odd because I seem to recall that I switched to Chrome from Firefox shortly after Chrome was released because it shipped with Dev Tools circa 2009.
I mean 5 years ago, Chrome added debugging Web Workers[0] so clearly Chrome Dev Tools were already quite mature by 2012.
My memory might be way off base here, but I'm fairly sure Chrome Dev Tools were in a reasonable state by 2012. Screenshots from the time certainly suggest so. My experience is much the opposite, in that it was around 2012 or so that I probably stopped installing Firefox on my dev machines altogether.
That said, recent features in Firefox (Containers especially) have me much more interested again.
I played around with Chrome in 2008 when it first came out in beta, and I remember finding Chrome Devtools an improvement over Firebug.
By the time the Firefox 4 betas were out, it was no contest; Firebug was very crashy (which, to be fair, it's hard for an extension to support a beta browser, but Chrome's dev and beta channels always had a reasonably stable devtools thanks to it being built-in).
As a beginner, it made debugging so much easier. Five years later, I still cant fathom using anything but Firefox, alas, without firebug.