Mozilla is rolling out two factor authentication for Firefox accounts and if you sync passwords using Firefox Sync you should enable it immediately.
The option for two factor authentication should show up in your Firefox account settings in a few weeks, but you can skip the wait by clicking this link. Do that and you should see the option for two-factor authentication, as shown above.
You’ll need to set up a dedicated time-based two factor authentication app for this to work. We recommend Authy, but Google Authenticator and Duo Mobile are also supported. SMS-based two factor isn’t supported, but you shouldn’t be using SMS for two-factor authentication anyway for all sorts of reasons.
We can’t overstate this: if you sync passwords using Firefox Sync you need to enable two factor authentication, or attackers could end up with access to your entire online life. Thanks to Catalin Cimpanu at Bleeping Computer for pointing this out to us!