How does this save money?

When using HireFire, you only pay for the resources that you actually require. For example, Heroku charges $25/mo for Standard 1X dynos (roughly $0.035/hour). If your application only requires 10 hours worth of processing per month then, under normal circumstances, you'd still be paying the full $25 if you let the dyno run 24/7.

Instead, using HireFire you can scale up and down automatically based on the amount of jobs that need to be processed. If your application only needs 10 hours worth of processing time, then HireFire will ensure that it'll only schedule dynos to run when necessary, reducing your monthly Heroku bill from $25 to $0.35 (10 hours * $0.035).