I triggered it a different way this morning using my STR in https://bugzilla.mozilla.org/show_bug.cgi?id=1755576#c0 Error Console showed me the same thing as comment 6.
Bug 1752307 Comment 11 Edit History
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I triggered it a different way this morning using my STR in https://bugzilla.mozilla.org/show_bug.cgi?id=1755576#c0 and then just closed the compose window and re-launched TB normally. Error Console showed me the same thing as comment 6.