Websites can compute a "fingerprint" based on browser data, such as the user agent string, screen dimensions, and other factors. If they do a halfway decent job, then your fingerprint will be unique or extremely rare. Usually these sites show you exactly what browser data they were able to extract.

It is possible to set some versions of Firefox to lie about various browser characteristics, but this creates a number of inconveniences to support the lie and I'm not sure it's worth it because if only a few hundred peoples' browsers tell the lie, your fingerprint will still be extremely rare.

The good news is that Tracking Protection blocks known fingerprinting services by default, so other than the test sites, you shouldn't expect to encounter it very often in the wild.

@jscher2000 thanks for the reply. It's just so frustrating. I have nothing to hide - but that doesn't give permission to websites to fingerprint and identify pinpoint and know everything.

EDIT: I just had a thought. The purpose of Firefox Focus is so that you are not tracked and traced. So they can't follow you with ads or track your behaviour etc. If they can STILL pinpoint who you are... then what's the point of it all??

I personally think the burden now falls on the likes of Mozilla to make browsers that reports blank information if the user so wishes - and to make this very very widely available and allow the user to do very very easily. (Otherwise, if there's just a handful of users using, then that in itself helps pinpoint them as they are the only ones hiding!) Just my opinion.