The option to attach files is greyed out.

This is a recently discovered bug. The symptom happens when you previously attached a file but now the previous containing folder no longer exists because it was deleted or renamed. When you later try to attach a file using the Write window's "Attach" button, it will silently fail, that is, clicking the button will appear to do nothing. The bug has been fixed for the next release, due in a few days. In the meantime, a workaround is to drag-and-drop the file onto the Write window using the mouse. Or, if you're comfortable delving into Thunderbird's Advanced Settings: - In <Settings | General | Config Editor> search for: "mail.compose.attach.dir". - The value (path) here must be changed to one that is still available (for example C:\). - The Attach button should work again right away - it's not necessary to restart Thunderbird.