I want the curved tabs we used to have – always standing upright; never hanging down – and I want them to display for all my tabs, focused or unfocused. I’ve reverted to FF v. 50 after a nasty experience with Tab Kit, so what would you all recommend? Thanks!
My advice is to stick to some newer version of Firefox. Either v55 stable or v52 ESR.
If you find some extension that does this it probably creates instability and will stop working in November since WebExtensions cannot modify the tab UI.
If I were you, I would ask on the FirefoxCSS community. Local CSS customizations will still work after Firefox57 in November, they will not eat up resources like legacy addons and a community has started building around them. There are some tweeks for tabs but none for Australis. If you ask there, maybe one of the CSS ninjas on that sub can help you out! 
Thanks Andrei.
I’m on 52.3 right now, your thoughts … ?
The link for the tweaks – that’s for Chrome unless I’m much mistaken (?)
It’s good that you updated to a supported version.
Firefox called it’s UI loooong before Chrome existed. From the description of the tweaks site “A community maintained repository of userChrome.css tweaks for Firefox".