If you opt in, they receive your email address, OS name, Firefox version, and IP; as well as encrypted sync data.
You send us different types of data depending on what features of the Services you use.
Email address: When you sign up for a Firefox Account, we receive your email address and a hash of your password. In addition, you may optionally add a profile image or use Gravatar (in which case, we'll share your email address with Gravatar to obtain your profile image).
Sync: If you enable Sync, we receive, in encrypted format, the data that you sync across devices (which may include Firefox tabs, add-ons, passwords, bookmarks, history, and preferences). While this cannot be decrypted by us, you should use a strong password to prevent unauthorized access to your synced data. We also store information about your device operating system and Firefox version to show you which devices are synced to your account.
Location data: For security purposes, we store the IP addresses from which you access your Firefox Account to approximate your city and country. We use this data to send you email alerts if we detect suspicious activity, such as account logins from other locations.
Find My Device: If you enable Find My Device, we receive the approximate location of your device only when you sign into your Firefox Account and specifically request us to locate a connected device. While signed in, you can see your device’s last known locations on a map. We regularly delete these locations and will not collect further locations until you request us to.