Well. It has been where it has always been. If you use the Firefox Menu Bar, put mouse top of browser and right click and tick Menu bar. This menu has been there since Firefox was called the other things, Phoenix and can not remember. Always there, Never moved.

You can also go to the 3 Bar Menu (far right side) and Click then Click Library and it is right there.

Now the wonderful thing https://www.howtogeek.com/333110/how-to-customize-firefox-quantum-and-remove-the-white-space-around-the-title-bar/ Put Library any where you want to in a open space, or move something to put it where you want. 3 Bar Menu then Optimize. Put other things on that page up or put things into the over flow if it to crowded. While there you may want to muse up to the Down Load Arrow and Click and un hide it. So make your browser, really yours.

More Wonder : 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

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". Note the Black Theme is just a Example and can be found in Customize.

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

I addition to what Pkshadow mentioned about displaying the Menu Bar all the time, you can hit the Alt key to reveal the Menu Bar temporarily, then select the corresponding letter key to open the drop-down menu. Then almost every menu item can be selected via a letter key. Works the same as it has done since Firefox 29, and there is no need to show the Menu Bar until toy need to access it. Very "user friendly", IMO.

Also unchanged, is using {Ctrl + H} to open History in the Sidebar. You can also open Bookmarks in the Sidebar using {Ctrl + B} and view Sync'd Tabs in the Sidebar, if you happen using Sync, and use Firefox on other devices.

There are a slew of Firefox functions that can be accessed via keyboard commands; many of which will work for similar functions in other programs installed in Windows. Real shame that more users aren't familiar with the keyboard commands, saves time when switching between various that serve those universal functions. http://support.mozilla.com/en-US/kb/Keyboard+shortcuts

You can find the History button in the Customize palette and drag this button to the Navigation Toolbar.