False positive result for IndexedDB test in firefox private browsing window

Firefox in private mode has indexedDB available, however it doesn't seem to be writable. `indexedDB.open` results in `InvalidStateError`. And when the `readyState` is `done`, the `IDBOpenDBRequest` object will have an error: `DOMError { name: "InvalidStateError", message: "A mutation operation was attempted on a database that did not allow mutations." }`.

I'm not sure what's a possible solution to this. If anyone can point me to the right direction, I'm willing to put together a PR to fix this.