You can do that in Table View. The menu at right sie has the option to show a 'select message' box by each message. If now using card view, click the icon at top of message list pane to make the switch

Thank you very much for the fast answer. I was not aware of it as I usually do not use the table view. I find the card view much more convenient.

Anyway, at least I know now how to get checkboxes and hope my wife will be happy with it.

BTW, does anybody know if this feature will be available in card view in future?

I have to ask to what purpose these check box would be used. They are supplied as an accessibility aid for those that struggle with certain mobility issues, but they server no purpose to someone using windows and a mouse. None at all. The Shift and Ctrl keys in conjunction with the mouse provide all the multiple selection options anyone could want especially in conjunction with the Ctrl+A select all shortcut.

I find almost everyone asking for check boxes is unaware of the features for multiple selection that come with their operating system and are supported by Thunderbird.

I'm neither unaware of using shift or ctrl in combination with a mouse, but there are others who can't due to mobility issues and there are also others who simply don't (want) use mouse and keyboard in combination (for what reason ever). And frankly speaking, when selecting multiple messages, just mark it with a checkbox is sometimes more convenient than using the keyboard in combination.

In table view it's available, in card view not? That does not make any sense to me.