Suddenly getting Firebase java.lang.IllegalAccessError at startup of app

According official doc:

No longer add the Android library com.google.firebase:firebase-core. This SDK included the Firebase SDK for Google Analytics. Now, to use Analytics (or any of the Firebase products that require or recommend the use of Analytics), you need to explicitly add the Analytics dependency:

com.google.firebase:firebase-analytics:17.2.2

So, No need to add com.google.firebase:firebase-core in your project. Remove it