Will it scale down worker dynos from i.e. 5 to 4?

There is an option (a checkbox) called decrementable in the web interface. When enabled, it will tell HireFire to adjust it's behavior to also downscale to non-0 quantities. This means that you can go from for example: 4 to 3 or 5 to 2 or 10 to 1.

If you don't enable this option, then HireFire will keep all of your worker dynos alive until the job queue size hits 0. Once 0, HireFire will reduce your dyno quantity to the configured minimum.

Decrementable

If you enable this option, it is possible that a worker dyno that's currently processing a job gets killed by Heroku and interrupts/terminates the job.

Preferably, depending on how important it is that every single jobs finishes successfully, you'd want to ensure that your worker library or custom implementation handles the SIGTERM signal properly, or implement a signal trap that handles it.