> why Firefox is not used to build more independent browsers the way chromium is?

Ease of integration. It's impossible to integrate Firefox compared to chromium. Unless this is solved, Firefox will die. The hope is new engines like Servo (maybe ladybird), where they are actually putting time and resources to make it easy to integrate. I'll never switch to chromiumia, but as soon as one of those new engines is mature enough, I'm definitely dropping Firefox.