Unfortunately “being the same everywhere” only works if your application is distinctive enough in functionality that it can’t be anything else but itself. Firefox is just a browser. Safari is also a browser. Chrome is also a browser. And none of these apps are more important or relevant than the operating system they are working under.

If my OS says windows and tabs close on the left, then I expect every single app to be a good platform citizen and not make me relearn default controls just to operate simple functions. Windows and tabs close on the left. End of story.