DOMException: A mutation operation was attempted on a database that did not allow mutations.

[Davidobot]

In firefox private browsing mode

[asiekierka]

This is caused by IndexedDB being severely nerfed in private browsing modes, both on Firefox and Chromium-based engines.

I work around this in the Zeta ZZT emulator by detecting exceptions thrown during IndexedDB initialization logic and switching to a plain JavaScript object-based, non-persistent database.