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Don't know about performance numbers, but one reason for headless browsers is so that you can run your tests on cheap small headless Linux servers.

Adding to ^. You don't need to have the Xorg stuff running to run tests.

That's what Xvfb is for.

"xvfb-run firefox" and "firefox -headless" should be functionally equivalent. But I would still be interested in performance comparison with numbers.

Doesn't xvfb still just run an Xserver, simply without painting? Or do headless Chrome and Firefox still require an X server running too?

You're right, there is a difference.

I meant equivalent from the point of view of the user who wants to run tests, not on the way they work internally.