It looks like Francine found a flamethower somewhere. This, depressingly, isn't all that uncommon; there's plenty of materiel leftover from the war just lying around if you know where to look. And of course, with Francine being modular, it's easy for her to attach things like this to herself.

She isn't actually shooting -at- anything. She's just kinda playing around with some targets she set up in the wasteland outside of town.