The Firefox Accounts team is in the process of releasing a new feature called Account Recovery. Previously, when a user resets their password, they would be given new encryption keys and could potentially risk losing any synced bookmarks, passwords and browsing history. With Account Recovery, a user can keep their encryption keys and not lose any data.

A more technical overview of how this feature works can be found here.

If you are interested in trying it out, simply goto your Firefox Account settings and click Account Recovery. If you do not see the Account Recovery option, you might not be in the rollout group yet. However, it can be manually enabled using these instructions.

From panel, click Generate, confirm your password and save the key displayed.

In the event you forget your password, you will be prompted for this key to recover your data.

Thanks and kudos to our security team, designers, developers, testers and everyone else that helped to make this feature happen!

https://blog.mozilla.org/services/2018/09/26/account-recovery-keys-in-firefox-accounts/