Does HireFire take scheduled jobs into account?

Yes. At least for the provided macros (Delayed Job, Sidekiq, etc). For example, the Delayed Job macro that's included in the hirefire-resource  library/gem takes the run_at  column into account by skipping (not counting) anything that's not ready to run yet.

If you're not using the hirefire-resource  library/gem, then you can create your own logic that doesn't count jobs that aren't scheduled to run yet when implementing the JSON endpoint.