Is this about a email address in Thunderbird or about an internet based mail service? Firefox: Use these steps to remove saved (form) data from a drop-down list: #click the (empty) input field on the web page to open the drop-down list #highlight an entry in the drop-down list with the mouse or cursor Down key<br>do not click the mouse or press the Enter key #press the Delete key (on Mac: Shift+Delete) to delete the highlighted entry ---- Thunderbird: Use AppMenu button - Options - Account Settings In the Account Settings window select the account to be removed in the left pane. Then choose 'Remove Account' from the 'Account Actions' drop-down menu at the bottom.