For UK Firefly 2 users.
The app will work on iOS if your device is in Airplane mode and Bluetooth is on but you can not be connected to internet. I downloaded the app on my older iPad mini by searching for Firefly in my purchase history. Whatever you do be sure to save this app on an old device so you can still change the temp, or change the temp to medium high and leave it.
This sucks, I actually just started using this vape again and was enjoying it. The app disappearing on both android and iOS is concerning, I hope it's just getting an update. It appears the reason the app is crashing is because it's connecting to a server or something when the app launches, I verified this by trying to launch the app while in airplane mode and with wifi on/off. When I had my wifi on the app crashed, wifi off the app worked as normal. Maybe the app is checking for updates at launch?