I'm sure people want me to release a theme to resemble Firefox's (relatively) new Proton theme, but I don't intend to, at least not at this time. But, I have put together a userstyle one can use with Photonic (the closest relative to Proton) to get a very similar effect:
https://raw.githubusercontent.com/Looty ... otonic.css
As something that I just threw together for fun, it looks pretty good (in terms of being close to what it aims to be, I aim not to talk about the design itself)!
I may do some more work on that in future, but for now it's out there, so feel free to give it a whirl.

The small icons mode will enable something resembling the compact density of Firefox, in the same way that Photonic does normally.