Firefox 57 has been released recently, hope you’ve liked it. Art Director of Mozilla, Sean Martell has released Firefox Quantum theme add-on and is available for Nightly and release versions. Yes! Separate Quantum themes available for stable and Nightly.
Firefox Quantum theme for release and Nightly versions
Visit the below links in Firefox 57 and click tap to preview, once you’re happy, install. Both themes can be installed in Firefox Quantum, and you can use any theme to your liking, for instance, you don’t needed to be using Nightly to use the Nightly theme.
After installing the theme, to switch to another theme, you can either visit themes panel in Add-ons manager or enter customized mode.