why has attachments stopped working

This is a known bug. https://bugzilla.mozilla.org/show_bug.cgi?id=2077526 Beyond drag and drop there is another workaround: * at the top right of the Thunderbird window, click the menu button ≡ > Settings > General * scroll down to the bottom and click 'Config Editor' * search for the preference mail.compose.attach.dir * the value (Path) here must be changed to one that is still available (for example C:) * restart Thunderbird That should fix it.