Not exactly sure what you mean by finishing off the firebrick. If you are talking about the sides of the firebrick showing around the opening, you can bring the faux stone all the way to the edge of the opening thereby covering the firebrick sides. However, if you do this (or even if you don't and just want to cover it with mortar) you shouldn't use regular mortar on the firebrick. Most building supply places have either pre-mixed high temperature mortar or fire clay which you can use to mix your own high temperature mortar. The fire clay bag will have directions on it (basically the clay, sand and cement). Regular mortar will eventually become brittle from the temperature swings and will crumble out.