We've seen what happens when almost everyone uses one browser engine (Internet Explorer): that vendor gets undue influence over the direction of the web, and their proprietary extensions make it difficult for others to compete.

We would end up in the same place if everything is based on Blink/WebKit; arguably we're partway there already.

Hopefully Servo will provide a competitive browser engine and full browser in the future as well, with the bonus of being great for embedding. But it's not quite there yet. And other potential competitors have their own problems. Firefox is, for the moment, the most popular and only viable browser not based on Blink/WebKit.

If what is left are Chrome forks, the Web is gone, replaced by Chrome OS Platform.

Without pressure from Firefox Chrome would have removed the hooks extensions need for effective ad blocking a long time ago.

More competition is better than less.

Firefox provides one of two browser engines.

If there's only one browser engine the Web immediately closes up like a giant impenetrable clam. All of humanity will be left outside, doomed to scrape forlornly on the shell with our nails and dream of the pearls within.

How is firefox not essential to the open web?