Firefox Accounts syncs your passwords, bookmarks and history across multiple devices. If you notice suspicious activity on your Firefox Account (like someone signing in from an unknown device), there are steps you can take to prevent further access to your information.
First, change your Sync password from a synced computer
This first step will prevent devices with your old Firefox Accounts password from connecting to the Sync server and getting all of the new passwords that you will have to create in the next section.
- Click on the menu button to open the menu panel.
- Click your Firefox Accounts (your email address or sign in), then click (or visit https://accounts.firefox.com).
- On the Firefox Account page, click in the Password section.
- Follow the steps to enter your old and new passwords, then click to finish.
Second, change all of your website passwords from a synced computer
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 using the existing already synced password.
See Create secure passwords to keep your identity safe for an easy way to create better passwords.