Mobile is where the war is being fought. A better browser can ship with hundreds of millions of mobile devices. Firefox-next with ad blocking and better support for parallelism is just what phones need. Users will use what came with the phone.
I'd love to use Brave on my desktop too, but their lack of plugin/extension/add on support cripples it a little. There's a couple I just can't live without. Using Iridium[2] on the desktop instead.
I believe Chrome has to be installed, but it clearly doesn't have to be the default. Just see all the Samsung phones with the default Samsung Internet Browser as evidence.
As long as Chrome is preinstalled, it will win.