Attach button not working on new emails

You're seeing a recently discovered bug - the symptom is that if you previously attached a file, but the previous 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 should be fixed in the next release. In the meantime, a workaround is to drag-and-drop the file onto the Write window. Or, if you don't mind changing Thunderbird's Advanced Settings: - In the 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:\). - Close and exit Thunderbird, then restart it (this step might not be necessary.)