Unfortunately, Mozilla took away the ability to consolidate bars in Australis, so Classic Theme Restorer is a *required* addon for this userstyle to work. (It lets you move the URL into the tab bar, and gives you the choice of curvy Australis tabs or the classic square tabs, which are a more efficient use of space. It also brings back the add-on bar and the original menu if you’re so inclined.)

I suggest you adjust the margins in the third and fourth lines to suit your system font size. Australis wastes A LOT of vertical space, so double your font size and that should be your negative margin size.
e.g., My system font on my main computer is size 16, so -32px yields a trim bar without cutting anything off.
e.g., My system font on my laptop is size 12, so -24px is more appropriate.

This is my personal style, and I’m all about the minimalism, so I may have stripped out something you want. If that’s the case, here’s how to get it back:

1) Vertical Scrollbars:
delete the first line: #content browser{margin-right:-15px!important;overflow-y:scroll;overflow-x:hidden}

2) Scrollbar Arrows (if you like scrollbars but just want to remove the arrows at top and bottom):
delete the last line: scrollbarbutton{visibility:collapse!important}

3) Favicons (the annoying little pictures on the tabs and URL bar):
delete #identity-box, and .tabbrowser-tab .tab-icon-image, from the sixth line

4) Min-Max-Close (I use Ctrl+W to close tabs and Win+Down to minimize, Win+Up to maximize, so these are just a waste of space for me. But if you want them back):
delete #window-controls, from the sixth line

5) Active Tab (I think it’s pointless to have the active tab highlighted in the tab bar, since it’s already displayed in the URL bar. Also, Australis enlarges the active tab by default, leaving a very annoying overlap that for the life of me I can’t figure out how to get rid of. So I decided to hide the active tab altogether, but if you want it back):
delete ,.tabbrowser-tab[selected=true] from the sixth line