Exactly the opposite; this is an evolution that has been taking place over the past decade.
Firefox has had tabs-on-top as the default since Firefox v4.0[1]. Chrome has had tabs-on-top for its entire existence (starting in Sept 2008); Opera had them before Chrome.
The merits can be argued either way, but don't act like this is sudden or arbitrary; it's neither.
Consider that maybe--just maybe--the dev teams at Mozilla, Google, et al. have done some usability studies in the past decade that informed these decisions.
I get that you may prefer tabs below the URL bar, but your claims about the greater state of UX are baseless and absurd.
[1] https://www.youtube.com/watch?v=HmgtW2Iw-kE posted June 2010
EDIT: Google made a comic for the release of Chrome 1.0 that includes an explanation of their original rationale for putting tabs on top. https://www.google.com/googlebooks/chrome/big_18.html