Many popular apps on macOS put commands under the Window menu of the form "Move to (monitor name)", e.g. "Move to Built-In Retina Display".
These commands make it easy for you to move the application window from one monitor to another in a multi-monitor configuration.
It is especially helpful when you work with the window maximized, or if you have difficulty using a pointer device.
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.
Isn't this more a job for the window manager? This should not need to be implemented by each application.