The severity of the sting is dependent upon species, how many spines were involved, how deeply the spines penetrated the wound, and the dwell time of the exposure (how long the spine was embedded in the wound).
First aid for a lionfish sting is immersion of the affected area in hot water (114°F) for 20 up to 90 minutes, or until the pain subsides, in order to inactivate the thermolabile components of the venom. The reason for applying heat to the wound is because lionfish venom is composed of heat-labile proteins, and the heat actually denatures the venom. Please, don't use scalding-hot water, as the resulting burn will likely do more damage than the venom. To ensure the proper temperature have a cooking thermometer on hand.
Don't worry, unless you happen to be allergic to the venom, or you get stung by a stonefish, your life really isn't in danger from the effects of the venom. That being said, you need to guard against secondary infection of the wound as well as make certain that there are no pieces of the spine left in the wound, which can cause infection. A tetanus booster is recommended if it is past due. To that end, you may want to seek professional medical assistance, just in case.
Foxface first aid is the same, altho foxface venom isn't as potent a lionfish venom.