Summary:
"firefly" may appear when typed in target in any command ("/tp", "/kill"...). Fireflies will not be officially added in 1.19 and, accordingly, there should not be such a target.

Steps to reproduce:

1. Open a chat.
2. Enter "/kill @e[type=firefly]"

Observed results:
You will be prompted to select "firefly".

Expected results:
The mob is not in the game and there should not be a command (target).

Image: