So you've just lost your phone (sorry about that) and you realize you've been using Firefox Sync to get all of your usernames and passwords on it. Here's what you should do to prevent someone from accessing your passwords:

First, change your sync password

This fist 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.

1. Click the menu button to open the menu panel.
2. Click your Mozilla account (your email address or sign in).
3. Click Manage Account (or visit https://accounts.firefox.com).
4. On the Mozilla account page, scroll down to the Security section and click the Change button next to Password.
5. Follow the steps to enter your old and new passwords, then click Save to finish.

Second, change all of your account passwords

This part is more time consuming but vital. If you don't change the passwords for things like your bank account, it would still be possible for someone with your phone to log into your account.