By default, Heroku only provides a system for "vertical scaling" (i.e. upgrading machines). But, they don't offer any system to add more machines when the load is getting bigger.
So, that's what Hirefire does.
To use Hirefire, just get into their platform:
Follow the configuration guidelines for the platform:
After that, you'll be able to defines rules on how to scale dynos depending on some configuration rules.
One common use case is to scale the number of sidekiq queue workers depending on the number of jobs waiting in that queue.
In that case, just configure the scaling:
minimum number of workers
minimum dyno quantity
maximum dyno quantity
And, you're done!