Firework rockets have the ability to be placed in the offhand, however, they cannot be used. I can't really be more descriptive of this, sorry.
Linked issues
is duplicated by 17
MCPE-186851
Offhand fireworks can't be used
Resolved
MCPE-176941
Fireworks do not work in the offhand
Resolved
MCPE-170288
Can't "use" off-hand rockets while flying with Elytra with Touch controls.
Resolved
MCPE-156312
Offhand and fireworks
Resolved
MCPE-100265
Firework rockets cannot be boosted in the off-hand
Resolved
Comments 3
If you have suggestions you can post them on https://feedback.minecraft.net/hc/en-us
Thank you for your report!
However, this issue has been closed as Invalid.
We're already working on achieving parity across versions where possible, and we're very keen to hear your requests or suggestions, so we'd encourage you to make a post at https://feedback.minecraft.net, or upvote an existing topic there.
If you're not sure whether your issue is a parity request or a bug, hopefully this article will help explain further: Parity Request Guide
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firework rockets are able to be placed in the offhand because of priority using crossbows as crossbows can fire fireworks if they are in your offhand they will fire instead of regular arrows