Each account can be configured to open the composer in text/plain or HTML mode. If the shift key is hold down while starting "write new message", "reply", "forward" etc. the preset should be inverted. This still works with the buttons in the header pane, but not with them in the Mail Toolbar nor with the menu (main, context nor meatball menu) items.
Bug 1829536 Comment 0 Edit History
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Each account can be configured to open the composer in text/plain or HTML mode. If the shift key is hold down while starting "write new message", "reply", "forward" etc. the preset should be inverted. This still works with the buttons in the header pane, but not with them in the Mail Toolbar nor with the menu (main and meatball menu) items. But context menu of the thread pane works fine.