Overwrite determines if other poison (or in your case fire) types are allowed to override/overwrite any possible already existing poison(/fire) types applied on the target.
Basically, tells you if it is allowed to replace other poisons.
Also overwrite is a boolean type variable IIRC, so only true or false (or 1 & 0, heard back in the day that is a better idea due to calculations and stuff).
The way it is setup is really stuffty, you can only have one type of poison effect at a time.
Using the script for a fire effect is... less then ideal.
Making a poison type for it would be even less ideal since then you could not be poisoned and on fire on the same time (yay for the players, less yay for gameplay mechanics).
At this point i am not really looking to rewrite it, maybe just some bug fixing, damage control here and there.
I have been through years of rewriting it (off an on), kind of stopped developing any mods nowadays as well, so i hope you understand my stance on it.
I will try to look into the emitter keeps going.
Have a feeling you might have missed one replacement of names however. Sounds like the image is not being removed properly so maybe you remove the poison image instead of the fire image.
As i said, will check later, but you might be able to find it based on this assumption.