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.
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.