The storage system is based on theWeb Storage API, with a few differences. Among other differences, these include:

• It's asynchronous.
• Values are scoped to the extension, not to a specific domain (i.e., the same set of key/value pairs are available to all scripts in the background context and content scripts).
• The values stored can be any JSON-ifiable value, not justString. Among other things, this includes:Array andObject, but only when their contents can be represented as JSON, which does not include DOM nodes. You don't need to convert your values to JSONStrings prior to storing them, but they are represented as JSON internally, thus the requirement that they be JSON-ifiable.
• Multiple key/value pairs can be set or retrieved in the same API call.

To use this API you need to include the"storage"permission in yourmanifest.json file.

Each extension has its own storage area, which can be split into different types of storage.

Although this API is similar toWindow.localStorage it is recommended that you don't useWindow.localStorage in the extension code to store extension-related data. Firefox will clear data stored by extensions using the localStorage API in various scenarios where users clear their browsing history and data for privacy reasons, while data saved using thestorage.local API will be correctly persisted in these scenarios.

You can examine the stored data under the Extension Storage item in theStorage Inspector tab of thedeveloper toolbox, accessible fromabout:debugging.

This page was last modified onJul 17, 2025 byMDN contributors.