Earlier quoted context omitted.
If Firefox dies, the open web dies. It's that simple. For the open web to remain open, there needs to be at least one more truly independent source of authority regarding how a rendering engine should work. Everyone else has thrown in the towel and abdicated that authority to Google by embedding Blink. Google is either actively malicious to the open web, or doesn't care about it other than as something they can strip…
Why does everyone always forget about Safari in these discussions? Safari has a respectable marketshare on mobile / tablets. Not as good on desktop but it’s not a lost cause.
So that's why.