''re : I've been through the process of App button/ View for a folder, but the option Sort By and the one below are greyed out.'' Whether you use the 'Menu Bar' toolbar and click on 'View' OR Whether you use the Menu app icon (3 horizontal lines) > 'View' The 'Sort by' and 'Threads' will be greyed out if you have selected eg: mail account name folder. You would need to select a folder first eg: Inbox If you do not want new folders to get the auto setting as 'threaded' then please check this as well. https://support.mozilla.org/en-US/kb/message-threading-thunderbird#w_changing-default-threading-mode * Menu icon > Settings > General * Scroll to bottom - click on 'Config Editor' * look for: mailnews.default_view_flags * Set value to 0 (zero)