• Operating System version: _____macOS Big Sur 11.5.2
• Browser version: _____ Firefox Developer Edition 92.0b6 (64bit)
• Firebase SDK version: _____8.3.1
• Firebase Product: _____ auth, firestore

Steps to reproduce:

I'm seeing the error "A mutation operation was attempted on a database that did not allow mutations" when starting my app in Firefox' private browsing mode. This was reported in #4069 and fixed in #4146 (8.1.2) so could this be a regression?

Exact error message:
Uncaught (in promise) IndexedDbTransactionError: IndexedDB transaction 'getHighestListenSequenceNumber' failed: InvalidStateError: A mutation operation was attempted on a database that did not allow mutations.