Works great, but from some reason the icon on main bar is in white, while all others are black. It is hardly visible for me. It seems the problem is caused by Pilot experiment Firefox Color.
You are right. In fact the whole process around "having an icon which fits nicely with the existing ones" is missing in the Mozilla API. With Firefox 58 there is at least some API to get notified about color changes and to get the new colors. I've added a hack to version 4.2.0 which dynamically creates an icon with the new color and sets it to the toolbar button. Please try the new version.
