Firefox uses unique identifiers to enable essential features like notifications, safe browsing and performance improvements. These identifiers are small pieces of data unique to your browser that help Firefox remember specific settings, such as which sites you want notifications from or whether a website is safe. While they don’t include your name or email address, they are considered personal data as they are unique to your browser and support features that enhance your experience.

Here’s a quick look at the main identifiers Firefox uses, along with a brief explanation of how each one works:

1. dom.push.userAgentID

• Purpose: Allows you to enable push notifications from websites that you subscribe to, to receive timely updates and alerts.
• Visibility: Found in about:config as dom.push.userAgentID.
• More info: Push Notifications in Firefox.
• Example: If you subscribe to notifications from a news website, this ID allows you to receive alerts directly in Firefox.

2. app.normandy.user_id

• Purpose: This ID helps Firefox assign users to experiments or new feature rollouts in a consistent, random fashion. Both the Normandy and Nimbus systems use this ID to decide which studies and features to activate. This ID is stored locally in Firefox and isn’t shared or sent to Mozilla.
• Visibility: Found in about:config under app.normandy.user_id.
• More info: Firefox Studies.
• Example: This ID supports experiments that help Mozilla gather feedback and understand how features perform across different user groups.

3. browser.newtabpage.activity-stream.impressionId

• Purpose: Helps Firefox track interactions on the new tab and search pages, allowing Mozilla to improve content recommendations based on popular features.
• Visibility: Available in about:config as browser.newtabpage.activity-stream.impressionId.
• More info: New Tab page in Firefox and Firefox Suggest.
• Example: When you interact with recommended content on your new tab page, this ID helps Mozilla understand which content is most useful to users. Also, if you disable recommended stories, Firefox sends a “delete” request to remove the impression ID and all related interaction data.

4. browser.contextual-services.contextId

• Purpose: This unique identifier helps Firefox manage sponsored content responsibly. It’s used to prevent fraudulent activity and allows Firefox to delete any tracking data related to interactions (such as clicks or dismissals) when sponsored content is turned off.
• Visibility: Accessible in about:config as browser.contextual-services.contextId.
• More info: Sponsored shortcuts on the New Tab page.
• Example: If you interact with sponsored stories or links on your new tab page, this ID tracks those interactions to help Firefox show relevant sponsored content and reduce the likelihood of fraud. If you choose to turn off sponsored content, all recorded data tied to this ID, such as clicks and impressions, is automatically deleted.

5. Telemetry Client ID

• Purpose: Supports Firefox’s telemetry service, helping Mozilla understand usage trends, identify performance issues and improve Firefox.
• Visibility: Accessible in about:telemetry#general-data-tab.
• More info: Telemetry Client ID.
• Example: Data from this ID enables Firefox to analyze usage patterns and enhance security and performance in updates.

6. usage_profile_id

• Purpose: This identifier is used to track general browsing patterns to help Firefox understand usage trends and improve overall performance. It is unique to your browser profile and is designed to support non-personalized data collection.
• Visibility: Found in about:config as usage_profile_id.
• More info: This ID is used locally and does not collect specific personal information; however, you can manage Firefox’s data-sharing options. For details, see Share data with Mozilla to help improve Firefox.
• Example: This ID may be used to identify common browsing habits that help Mozilla optimize Firefox’s performance for tasks that most users frequently perform.

7. Telemetry Profile Group ID

• Purpose: The Profile Group ID serves as a general identifier across Firefox installations, allowing Mozilla to group profiles for features and studies without assigning a unique identifier to each profile. This ID supports Firefox’s ability to roll out updates or test features across various user segments.
• Visibility: Accessible in about:telemetry#general-data-tab.
• More info: Firefox Studies.
• Example: Data from this ID allows Firefox to analyze usage patterns across grouped profiles.