You can lock your screen on Windows by holding down the "Windows" key and tapping the L key. I once worked at an office where if a co-worker saw that you left your screen unlocked they would send an embarrassing email from your computer, so I habitually lock my screen -- even alone at home.
For Sync, you can add multi-factor authentication to your Firefox Account so that someone trying to Sync needs to enter a code before their connection request will be approved: https://support.mozilla.org/kb/secure-firefox-account-two-step-authentication
