The Dev Tools, Options and Config buttons on the right of that screenshot appear to be added by an extension. Possibly [Phoenity Buttons](https://addons.thunderbird.net/en-US/thunderbird/addon/phoenity-buttons/) >- It may be necessary to restart Thunderbird for buttons to move or appear. Not sure why it wasn't there before. I have added the Add-ons button to the Mail toolbar from Customize, but don't see the others there. If they don't move after a restart I would suggest using the Support Email address in the extensions page and also report the issue with the developer.
Bug 1664634 Comment 1 Edit History
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The Dev Tools, Options and Config buttons on the right of that screenshot appear to be added by an extension. Possibly [Phoenity Buttons](https://addons.thunderbird.net/en-US/thunderbird/addon/phoenity-buttons/) ~~>- It may be necessary to restart Thunderbird for buttons to move or appear.~~ >Currently only a few (unmovable) buttons are available: >- Search Messages, Config Editor, Developer Tools, Preferences & Restart. Not sure why it wasn't there before. I have added the Add-ons button to the Mail toolbar from Customize, but don't see the others there.