Visual design overhaul for Firefox or new theme for Mozilla browser, Australis has been landed on Nightly and available for testing since few days. New design ships with curved tabs similar to Chrome. If you don’t like these curved tabs and ok with other interface changes Australis brought then you’ve an extension available on AMO which changes curved tabs to Rectangular tabs.
Change Firefox Australis Curved Tabs to Rectangular Tabs
Open Nightly and install Firefox Tab Mod extension and restart browser. Your browser now shows Rectangular tabs instead of curved ones. If you want classic or old theme for Firefox back then consider installing Classic Theme Restorer add-on which not only restores old squared tabs, but offers options to bring tabs to bottom and enables add-on bar and offers other tweaks.