The file name for version 1.0.4 is
fireproofboats-1.21.0-1.0.4.jar on Modrinth, while it is
fireproofboats-1.21.1-1.0.4.jar on Curseforge
This also applies to the manifest file that differs by a singular byte.
I am not sure if it is intentional or not but notifying just in case.
I noticed this after trying to import a my Curseforge modpack into Modrinth and looking through all the mods it did not recognize, this being one of them. After downloading it from Modrinth I noticed that it created a duplicate so I looked into why and this was the result. I have not found any other issue connected to this difference.