5% faster page load still isn't a killer feature.
>Extensions were amazing
And the reason everyone who left used the browser.
>lose extensions and start over.
A smart company would never have done this. They would have maintained both to see if anyone would use the new browser. Instead, they took their customers on a forced march down a trail of tears.
Imagine if another company tried this. Imagine if Tesla said, "We know you love electric cars. So today we're going to stop making them and sell hydrogen powered motorcycles instead. They're faster! Sorry, we won't be making parts for or supporting the cars anymore." It would be corporate suicide. Imagine how angry the car owners would be, after they invested in the idea and the charging equipment, only to be abandoned by the company.
It makes perfect sense to continue shipping the old product in the face of a "new better" one. Apple continued selling iPods long after the iPhone arrived. Killing the successful core product and releasing a new untested competing product is an insane gamble. I can't see how Mozilla leadership ever thought that was a good idea.