Message managers provide a way for chrome-privileged JavaScript code to communicate across process boundaries. They are particularly useful for allowing chrome code, including the browser's own code and extension code, to access web content when the browser is running web content in a separate process.
These guides explain how to use message managers in multiprocess Firefox.
Note that none of this requires multiprocess Firefox: everything described here will work with single-process Firefox, so the same code will work in both variants.