[Wilya]

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.

[Vinnl]

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

[Wilya]

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.