>Why is Firefox losing marketshare

very simple answer, because Mozilla doesn't control the infrastructure that runs on 80% of smartphones in the world and ships Firefox as the default browser.

It really has nothing to do with the bespoke features that people on HN pay attention to. Firefox doesn't control any platform and defaults matter. There's a reason Google pays them a gazillion dollars to be the standard search engine, which you can change with one click. It's also why Safari is still going relatively strong.