FFZ is a client-Side application that runs in the context of the Twitch Website thanks to how modern Browser Extensions work. This allows it to have access to the user’s credentials and act on the user’s behalf without further authentication, effectively giving access to all data the twitch page itself has access to.
This allows it to poll such information with relative ease, but this is an undocumented (and thus unsupported / may break at any time) approach.
If you want a local solution, you could build a simple script that prompts for an implicit oauth token and uses that to access the api, manually renewing said token once every 2 months when the token expires.
If you are curious about FFZ, you can find its source on github: https://github.com/FrankerFaceZ/FrankerFaceZ