Isn't it obvious? Microsoft's former monopoly was eroded first by the web, then phones, then tablets. Using a non-Microsoft computing platform is nowhere near as restricting as it was in the 90s. And they never got the same foothold in newer markets they entered since then.
Firefox 2 and the iPhone came out about the same time. I think the one-two punch (and you can argue which is 1 and which is 2, but I'm not interested) was that on one side you had a device, populated by people with considerable disposable income, that couldn't run IE if you tried. That got the attention of management in ways us developers never could.
And on the other, you had a safe harbor for people who were up for a switch elsewhere. And it helped substantially that debugging was better on that alternative (developers would use Mozilla and Firefox for development whether management wanted FF support or not).