Firefox Extension Data Access: Can Extensions See Each Other’s Settings?

TL;DR

Generally, Firefox extensions cannot directly read preferences or data stored by other extensions for security reasons. However, there are limited ways they can interact, primarily through the browser’s APIs and shared storage areas (like local storage), but these interactions require explicit permission and cooperation from both extensions.

Understanding Extension Isolation

Firefox is designed to isolate extensions from each other. This means each extension has its own separate environment for storing data, running code, and managing preferences. This isolation prevents malicious or poorly written extensions from interfering with others or accessing sensitive user information.

How Extensions Store Data

Extensions typically store data in one of the following ways:

• Extension Storage API: This is the preferred method. It provides a key-value storage system specifically for extension data. Each extension has its own isolated storage area.
• Local Storage/Session Storage: These are web storage APIs that extensions can use, but they’re shared with any website running in the same browser profile. This is less secure than Extension Storage.
• IndexedDB: A more complex database system for larger amounts of data. Each extension has its own isolated database.
• Cookies: Extensions can access and modify cookies, but this is generally used for communication with websites rather than other extensions.

Can One Extension Read Another’s Data?

Directly accessing another extension’s storage area is not allowed by default. However, there are a few limited scenarios where interaction is possible:

Steps to Enable Limited Interaction (If Possible)

1. Message Passing: Extensions can communicate with each other using message passing through the browser’s runtime.sendMessage API. This requires both extensions to explicitly listen for and respond to messages.

   // Sending a message from extension A to extension B
   browser.runtime.sendMessage("extension-id-of-b", { type: "getData" }, function(response) {
     if (chrome.runtime.lastError) {
       console.error("Message failed:", chrome.runtime.lastError);
     } else {
       console.log("Received data from extension B:", response.data);
     }
   });
   

   Extension B would need to listen for this message and send back the requested data (if it chooses to).

2. Shared Storage Areas (Local/Session Storage): If both extensions use local or session storage, they could potentially access the same keys. However, this is strongly discouraged due to potential conflicts and security risks.

   // Example of using localStorage in extension A
   localStorage.setItem('myKey', 'myValue');
   const value = localStorage.getItem('myKey');
   console.log(value); // Output: myValue
   

   Be aware that any website can also access data stored in local/session storage, so it’s not a secure way to share information between extensions.

3. Content Scripts and Shared DOM Access: If both extensions inject content scripts into the same webpage, they can interact through the shared DOM. This is limited to modifying the page’s content.
4. WebExtension APIs (Limited): Some WebExtension APIs allow for controlled interaction between extensions, but these are typically specific to certain functionalities (e.g., context menus).

Important Security Considerations

• Permissions: Extensions must declare the permissions they need in their manifest.json file. Users will be prompted to grant these permissions during installation. Accessing data from other extensions requires explicit permission, which is rarely granted automatically.
• Content Security Policy (CSP): Firefox enforces a CSP that restricts what resources an extension can load and access. This helps prevent cross-site scripting (XSS) attacks.
• User Privacy: Always respect user privacy. Do not attempt to access data from other extensions without their explicit consent or a legitimate reason.

Checking Extension Permissions

You can view the permissions an extension has requested by going to about:addons in Firefox, finding the extension, clicking on ‘Manage Extension’, and then looking at the ‘Permissions’ section.