Despite great progress in fuzzing browser APIs, systematic approaches for testing web storage techniques remain absent. IndexedDB, the most popular NoSql database in modern browsers, brings unique challenges for fuzzing its API due to its asynchronous event-driven feature and strict phase separation. Current browser fuzzing techniques frequently struggle to generate nested event flows and invocations, which significantly impacts semantic correctness. Moreover, they often rely heavily on try-catch block to suppress exceptions, which introduces substantial performance overhead. We propose IDBFuzz, the first fuzzing approach tailored for the IndexedDB API, which effectively tackles the challenge of capturing the execution context and event semantics inherent to IndexedDB, as well as handling large persistent objects. We design a seed generator based on intermediate representation (IR) that decouples layered IR skeletons from input object generation. With the aid of a global database snapshot, IDBFuzz can generate semantically controllable seeds, enabling the efficient production of high-quality test cases that significantly improve coverage.