InitializationException in Unity Firebase
So, after 1245321653214th attempt to reimport Firebase and to resolve this issue anyhow we discovered that previously the other plugin, Admob, overwrote some libraries that Firebase uses.
We removed all aars and jars both Firebase and Admob may use and firstly reimported Firebase and then, selectively, Admob. The main rule is not to allow Admob overwrite any files Firebase uses, for example "play-services-blah-blah.aar".
Main piece of advice in this post is not to hurry with importing plugins: import one, configure, build, check if it works (10 times), commit, import the seconds one and so on.. This way you'll clearly see if 3rd parties have confilcts between them.
If you are already in the mess, you should better delete all 3rd parties and start from scratch.
So, be VEEERY careful with files similar to those on the image below.
Hope this helps people to prevent problems we had.
I was having the same issues as described in my comment above.
Have a fully working project with unity 2018.3,8f1 and firebase 5.4.4. Upgraded unity to 2019.2.6f1 which forced me to also upgrade firebase. I upgraded without too many issues, but then get this same error. Tried all of the solutions here, but get no results. I assume this is due to some conflict with other plugins as well, but can't figure out which ones. (also firebase was the last one installed, so I would expect it to overwrite any other issues)
I only had these modules installed: FirebaseAuth, FirebaseMessaging, FirebaseRemoteConfig, FirebaseAnalytics.
I then installed the FirebaseCrashlytics module, and it fixed my issue.