So this is the replacement of Personas Plus. Well it's not as good, that the least to say.
- On its customiser page, Personas Plus had a very convenient colour picker. Firefox Color hasn't. One star disappears.
- While the interface is efficient and easy to use, I could only use it once. Indeed, from the time I saved the theme, I couldn't go back to the interface, neither with the add-on button nor the URL. I only get a flash of that quote page and that's it. One other star disappears.
- If theme is changed via add-ons menu, where by the way saved theme from F Color isn't even listed, you can reload this saved theme by clicking the add-on button, only if all F Color tabs are closed. One other star disappears.

Maybe the problem comes from my OS which is Linux Mint 19.1 Cinnamon 64-bit.

Personas Plus has less fancy options than its successor but at least still works to apply a custom theme. I stick to it for the moment.