To preface, Yeeps has a ton of movement tools that can send you higher for menial amounts of stuffing. The following information isn't to request nerfs to these items, but to describe the bar that fireworks need to reach, and I don't think the firework is in need of being changed functionally.
The grappling hook is a very reliable and cheap option that costs 300 stuffing to get out and 200 to attach to an object. Doing so allows you to pull yourself closer to the source, launching you distances that greatly outpace what the firework is capable of, and can be followed up with another grapple.
The recently added fire extinguisher also totals at 500 stuffing and is far more controllable, interacts with fire, and lasts for a much longer period.
On the side since it synergizes with all of these, hitting yourself with a snowball allows you to take the momentum each of these give you which in most cases brings you to world height consistently. The cost of this is an additional 300 stuffing and actually makes the grappling hook stronger by usually removing the need to reattach as the momentum you get is so strong.
With all of this in mind, the value the firework can get you is peanuts for 600 stuffing to launch you a distance you can barely react to or support yourself with, leading to a vacancy of usage with the playerbase as there are far better options.
My ideal cost for the firework would be in the range of 200-400 as the grappling hook and fire extinguisher both at a base cost of usage are 500 for far more mobility, letting it emerge as a quick oneoff use item to quickly go off in a direction.