How do I make a fire break around our chimney?
Prefabricated chimneys have specific firestop spacers that must be used to maintain the terms of their listing, though they are often just sheet metal with a hole cut in it.

For constructed chimneys such as those like yours of masonry units, use firestop foam to fill the gap between framing and the masonry at every floor and ceiling level. This stuff is made by many of the same companies that make other materials used in construction. It's typically much like expanding polyurethane foam used as a sealant to fill gaps in construction, except it contains chemicals so the material will not support a flame, the stuff just chars. Regular foam will support a flame and puts off noxious fumes when burning.