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!