May want to read these first ; How about looking at these 2 URL's first : https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/ and The name is not descriptive enough : https://www.howtogeek.com/332449/whats-new-in-firefox-quantum/

More information will come out slowly. New extensions are being made and some older ones being updated that can track here : http://bit.ly/2j0Anlj

Going back : unfortunately 56.0.2 is no longer safe to use for every day use, but 52.5.0 ESR is! It will continue to get security updates until May 2018, and you can download and install it from this page: https://www.mozilla.org/en-US/firefox/organizations/all/ Note : Legacy Extensions will be deleted or removed in any version update after May 2018.

You should make a backup of your Profile before going back and just because: https://support.mozilla.org/en-US/kb/back-and-restore-

Please let us know if this solved your issue or if need further assistance.

Update : It isn't completely possible to use the Australis design with Firefox 57, but depending on what you want, you might be able to customize the UI to look more like the old one:

   This project: https://github.com/wilfredwee/photon-australis lets you have the old tab bar appearance back in Firefox 57.
   You can remove the space around the address bar by right-clicking on the spaces, and choosing "remove from toolbar". 

Here's a more comprehensive project which affects not only tabs but brings back the old "grid-style" options menu. https://github.com/axydavid/FirefoxUI/blob/master/README.md

Note the Black Theme is just a example and it can be found in Customize

Please let us know if this solved your issue or if need further assistance.

Thank you! I'll study. I have installed the firefox 56.0.2 and am very pleased. I turned off automatic update