I have been using it since 2006 (Firefox 2.0). Back then, the presence of the Firebug extension for Firefox made me switch to Firefox as my primary browser.
Two years later, Chrome arrived but Firefox remained as my primary browser because I wanted to continue using the Vimperator extension for Firefox that allowed mouseless browsing with Vim-like key bindings and commands.
Firefox still remains as my primary browser with Chrome being an additional browser that I use sometimes. These days, I use the Vimium extension for both to use Vim-like key bindings. I occasionally use Safari and qutebrowser too.