So you've just lost your device (sorry about that) and you realize you've been using Firefox Sync to get all of your usernames and passwords on it. This article describes what you should do to prevent someone from accessing your passwords on your lost device.
This first step will prevent your lost phone from connecting to the sync server and getting all of the new passwords that you will have to create in the next section.
This part is more time consuming but important. If you don't change the passwords for things like your bank account, it would still be possible for someone else to log in to your account on your lost device.
See Create secure passwords to keep your identity safe for an easy way to create better passwords.