fire blocking separates the air between structural cavities. Floor and ceiling cavities can share air space with attics and crawlspaces but the air must not be transferred through a wall cavity to another floor system of the home. look at how a typical wall is built with the bottom wall plate and upper top plates, the plates are the fire blocking basically because they stop air from the bottom of the house to the top and all floors in between. if you have to install blocking inside of wall stud bay then do the blocking just like a wall plate, it's just cut into single stud bay pieces instead of a full length plate as normally used. check with the tradesmen before adding your blocking. in general it is better to have the blocking installed before mechanical and electrical but some spots the tradesmen may need access to before you plug the spot with blocking,so check with them about it. otherwise the blocking is always in first and then tradesmen show up to drill holes and notch the blocking to pieces and then the firestop is added