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-53490
Fireworks don't work in offhand
Resolved
MCPE-56250
When I put fireworks in my off hand i cant use them to launch my elytra.
Resolved
MCPE-64473
Firework rockets do not activate in off hand (1.14.30)
Resolved
MCPE-71125
Fireworks become unusable when put in off hand
Resolved
MCPE-75276
Fireworks In Off-Hand Don't Activate
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