If you want to see if your Firefox account is used on another device, you can do this by :

- Clicking on the Firefox Account icon in the toolbar.

- Clicking then on Manage Account.

- Selecting Devices & apps

If you see there that only your current device is linked to your Firefox account then you will know that no one else is connected to your account. If some other devices are linked to your account, you can disconnect them and revoke their permissions from the Devices & apps menu.

Changing your password will then make sure that nobody else can log in into your account using your former password.

Check the following links Manage the devices linked to your Mozilla account & Suspicious activity on my Firefox Account for more info .

Note that Sync isn't meant to be used as a backup but is meant to sync data among other devices, so if you only connect one device then not much will happen. The data is on the Sync server, but it isn't guaranteed that you can use it in case you lost data on your device. See also:

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles