Security of Firebase Connector or API REST integration


I am about to publish my Android and iOS app. In my project I need to access Firestore data and firebase functions using API key. This question is really concerning me about the security of my app.
Is it secured enough to have my API key in the Firebase Connector or in REST API as “key” parameter? I am only doing mobile app.
If not, what’s the best way to do it. I saw some topics talking about this but there are always some questions remained in the corresponding topics.

Thank you,