Firebase can be used, you just need another method
This might be a rash move, you would probably have to re-write all your projects from scratch, and from what i have seen the firebase realtime database implementation on kodular is not the easiest thing to work with.
You would need to completely remove the firebase component from your project to remove the error messages
None of your extensions have recently shown to generate this error.
What can you do?
Use the Firebase RealtimeDatabase REST api with the AppInventor Web component.
Search through extensions to see if there is a suitable firebase extension that meets your needs
Switch to using CloudDB, AppInventor's own no sql offering
If you feel like doing everything yourself, but still want to work with json nosql, perhaps consider trying infernus