There are a number of inconvenient side-effects to firebreathing, from chemical pneumonia and varying degrees of cancer, to just plain setting your face on fire. One of the more embarrassing problems however are the dreaded paraffin burps.
Inevitably, when you're quaffing liberal amounts of fuel and spraying gouts of burning hydrocarbons into the air, you will swallow small amounts of whatever you're using (Hopefully not too much though; paraffin runs are an entirely different kettle of fish altogether). This will hang around for the next few hours and will occasionally repeat on you, not unlike that last chalupa you had. Unfortunately, unlike the delicious taste of Mexican food, you will have a tide of greasy, oily flavour wash over your tastebuds, something akin to performing fellatio on a petrol cap.
The taste will also linger in your mouth for sometime, complete with an oily residue, which can only be purged by some sort of beverage, alcoholic or otherwise (Although I'm not advocating playing with fire after a few beverages; friends don't let friends firebreath drunk). Left unchecked, it will continue to flavour your mouth in a similar way that garlic does, so unless your partner also casually chuggs paraffin, they probably won't appreciate sloppy makeouts afterwards. Have a pint of milk to settle things out, and everyone'll be happy.