Proton is Firefox’s new design, starting from Firefox...

Proton is Firefox’s new design, starting from Firefox 89.
Photon is the old design of Firefox which was used until version 88.

Proton’s overall feel is good, but there were a few things I didn’t like and wanted to improve.
That’s why this project was born, and Lepton to denote light theme layer.

Disclaimer: It works on Firefox 89 and above!!