Oops, you switched off the menubar, too. So then press the Alt key to temporarily show the menubar again.
Bug 1929066 Comment 7 Edit History
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Oops, you switched off the menubar, too. So then press the Alt key to temporarily show the menubar again. Stuff like this should really be posted at Thunderbird's support forum: https://support.mozilla.org/en-US/products/thunderbird