There used to be fireflies in the jar, but now there aren’t anymore, for some reason.

The jar is still a source of light, though. This is just a visual glitch.

Minecraft version: 1.12.2
Forge version: 14.23.0.2531
The Twilight Forest version: 3.4.239

Petition to rename "Firefly Jar" to "Jar"

In all seriousness try checking your particle settings, if you have particles set to off or minimal they might not appear.

Particles are enabled.

Will add parameter to force particles on no particles to the method

@Tamaized Thanks! 👍