Firefox doesn't do what I've set it to do with downloads

I've set Firefox to open PDF files in the PDF viewer that I use, and I've set it to "do this automatically for files like this from now on," yet every time I go to download a pdf, I get the message window popping up asking what I want to do with this file. I've gone through the troubleshooting of deleting the handlers.json file that the Support archives recommended trying, but it hasn't changed things.