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It makes what claims, but if by mistake you open another window, it will not close...

Это отзыв для предыдущей версии этого дополнения (1.0.0.1). 

This issue has been fixed in version 1.1.0.0, which is awaiting its review by the Thunderbird add-ons team since about a month now. If you don't want to wait for the review, you can download it at https://addons.thunderbird.net/thunderbird/downloads/file/1016729/minimize_on_close-1.1.0.0-tb.xpi