I'm of the opinion that with the January patch that makes Jason immune to most forms of stun in rage mode, the value of firecrackers has been significantly reduced.
When playing as Jason I love when councillors throw firecrackers. I intentionally step on them to increase my rage metre. The stun seems to only last about 3-4 seconds and in most cases doesn't really hinder my ability to kill a councillor. It only delays me slightly.
Before the patch I found firecrackers useful for stunning Jason at the exit to reach the cops. Now most of the time when the cops arrive Jason is already rage mode and this tactic is no longer possible.
It seems the only time firecrackers come in handy is when your trapped in a tight space (e.g. small cabins) and Jason is closing in on you. Other than this they feel like a useless item..