The Mozilla help only explains changing the items that exist. I cannot add CSV to the list in order to tell FF how to handle.
But if it is currently doing something with the csv file then it should be in the list already, probably set to “. That’s certainly how it works for me. Then you can change it to “Always ask” or similar. In my FF it is listed, under “c” for csv. There are a couple of versions of csv files there.