If you're using Sync, that's how it works. You change your password for Google at home - that change is almost immediately sent to the Sync server; then the next morning you open a different device that is using that same Sync account, and quicker that you can even open Options > Security --> Passwords to look at your password data, that change of data is synchronized into that device. The only way that a "hacker" could get that information out out Sync, is if he was connected to your Sync account as an additional device. That hacker would have needed to have physical access to one of your devices and to Firefox on that device you get your Sync account information to begin with or pulled it out of the air via an insecure WiFi connection you were using at the same time. And if he succeeded in connecting to your Sync account, all your passwords that were saved in Firefox would be exposed - not just one for Google. I suggest that you disconnect both devices from the Sync service one at a time, and then when they are both disconnected go here - https://accounts.firefox.com/signup - and change your account password. Then using that new password, reconnect to the Sync service.