Puntuado con 5 de 5 estrellas

Thanks for answering,

Do you have in your plans to make it to work with FF 57?

Esta valoración es de una versión anterior del complemento (0.5.5).  Este usuario tiene una valoración anterior de este complemento.

If the WebExtensions API doesn't support it, nothing I can do.

As long as FF keeps support for UserChrome.css, you can still manually add the CSS (just stick the mosaic in the same folder as userchrome.css).