Definitely not, at least in the case of front-end devs. Speaking as one myself, part of this is that Chrome's dev tools are so much better. Part of it is that the great majority of your users are on Chromium, so that is the most important case to test. But of course it's a self-reinforcing cycle: the more devs that only use Chrome, the more apps that work best on Chrome, the more users that switch over to Chrome, the more devs who only feel the need to test on Chrome, repeat.

That depends. CSS prototyping is usually easier in Firefox. For example grid editing, flexbox, shape path editing.

I mostly switch to Chrome when debugging PWA. Other then that Firefox gives me better UX.

It's recommended to install your goalposts with concrete footings that go below the frost line to prevent movement.