You can change the Sync password: *https://support.mozilla.org/kb/ive-lost-my-phone-how-do-i-deactivate-sync You should always do a full clear and wipe all data if you sell a computer. *http://www.thefreecountry.com/security/securedelete.shtml Did you at least remove the Firefox profile folder? *http://kb.mozillazine.org/Profile_folder_-_Firefox Vista and later: *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\ *C:\Users\<user>\AppData\LocalMozilla\Firefox\Profiles\<profile>\ Windows XP *C:\Documents and Settings\<user>\Application Data\Mozilla\Firefox\Profiles\<profile>\ *C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Profiles\<profile>\