alt Hacker NewsIt's hard to state just how much of a game changer Firebug was for web development. Before that your only option was "alert()"ing or outputting directly to the page.
Once Chrome came along with their devtools, improvements quickly escalated between the 2 before Google eventually won out.
I can't recall the exact point in time when my use of Firefox fell off, but it was probably due to the account integrations with Chrome.
Around 2006 or 2007, I was working on the Sage Notebook. I did a little JavaScript injection, and managed to make the notebook execute JavaScript instead of sending Python code to the server and printing the result. Lo and behold, I could interact with my JavaScript environment on any browser we supported (ie, ff, safari and Opera). I don't recall if firefox had its JavaScript console yet but it was a game changer on those other browsers.