How to get curved tabs in Firefox 57

You can change Firefox 57 Quantum to look like the old Firefox with curved tabs by using the developer style sheets.

Photon Australis: Bringing sexy curves back to Firefox Photon.

How do I get this sexy thing? 😍

Since Firefox 57+, the only way to customize your browser UI is through userChrome.css. Learn more here.
Rough instructions:

1. Go to your Firefox browser, type about:profiles in your url bar.
2. Under the profile that is in use, open the Root Directory folder.
3. If that folder does not have a chrome folder, create a folder, name it chrome.
4. In the chrome folder, create a file named userChrome.css
5. Copy and paste the code that matches your Firefox theme. Dark, Default, Light
6. Restart Firefox and enjoy some sexy curves :D
   
   Source: https://github.com/wilfredwee/photon-australis