Do you recognize having logged on to Sync during that time ? Have you ever used a proxy or VPN ? Is you aren't sure, then you should consider to change the password and maybe also do a malware check. *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware You can check the connected devices on the Firefox Account Settings page and possibly disconnect some older or unknown devices. *https://accounts.firefox.com/settings