The problem is that it isn't.
Do you know why Firefox managed to usurp IE6 in the first place? Because it won the adoption and appeal of tech enthusiasts and professionals. Mom and pop (read: the general population) switched to Firefox from IE6 because their tech nerd kids installed it for them, and the enterprise largely moved off of IE6 dependence because the general population moved off.
But the Firefox today is not the Firefox that defeated IE6. Mozilla steadily eroded and destroyed every single thing tech enthusiasts and professionals loved about Firefox, to the point it practically became just a Chrome ripoff. At that point, why bother? Chrome's right there, the real deal.
Not to mention Mozilla happily takes money from Google with no shame at all so their CEO can get her fat paychecks.
Firefox is not a viable alternative, Firefox is literally controlled opposition to pedantically argue Chrome is not a monopoly. Not even the Intel and AMD x86 duopoly is this blatant.
