> I want to be able to modify/add code

Yes, but I want to do that in my editor, not in the browser.

> try it out, then rewind, and try something else. Then save my changes once they work perfectly.

Certainly I do not want to copy'n'paste the code then back from browser to my editor.

Let me all do this in my editor of choice. The browser is only where I execute stuff, a debugger belongs into the editor/ide.

FWIW, I think it's planned by debugger.html project that the debugger can be used by different editors/IDEs. See the presentation at React Rally (relevant snippet is at https://www.youtube.com/watch?v=Fk--XUEorvc&feature=youtu.be...) showing the integration into emacs or even using debugger.html as CLI tool.