You can just bring up an email sheet in your app, pre-filled with your email address and a subject. No need to use Firebase. It can be nice if you don't have too many users, but can quickly become too much to handle if your app gets popular. In my apps I have a FAQ section that users have to scroll through first to get to the button to email me at the bottom.

@hendrik@home.social
Indie app developer (Pocket Pixels Inc), creator of photo editing apps Juxtaposer and Color Splash.
I thought it did that already since iOS 6? https://www.macstories.net/news/ios-6-messages-now-automatically-selects-last-used-international-keyboard-for-each-contact/
But I use a third party keyboard, maybe you do too?