You cannot use Shift-Click twice for a given set of messages. Use Shift-Click once, and then use Ctrl-Click for any remaining messages to be selected.

Alternatively use Shift-Click to select a consecutive set of messages, and then use Ctrl-Click to de-select the ones you don't want.

Thanks for the quick response.

This is surprising and disappointing. Pretty much every app I've used that has a list allows repeated combined use of both Shift-Click and Cmd-Click (Ctrl-Click). It's a major shortcoming, particularly in such a mature and fully featured app.

I'm new to TB and I'm importing large mailboxes. The absence of this basic selection method will add many hours to my workload. I'm guessing that there are no workarounds?

This behavior isn't specific to Thunderbird, it's imposed by the OS. I have no idea about Mac OS though, may be it's different there.

I think that TB is the only app I have that doesn't allow repeated use of both Shift-Click and Cmd-Click (Ctrl-Click) while maintaining the selected items. As far as I know, this is standard behaviour across the board.

In my previous mail client when cleaning up a big list I'd sort by sender, Shift-Click to select the groups to trash or move & then maybe sort by subject, select those groups & so on. At each stage, the selection remained & I could add more individually with Cmd-Click (Ctrl-Click). Simple and efficient.

I don't know if that behavior is OS or app based on a Mac but as it works on everything (maybe there are exceptions but I'm not aware of any) it appears that it's TB that is specifically set up to disallow it.