Aha, I forgot that in Thunderbird and other email clients, one needs to click a message to see it in full and read it (right). So then Thunderbird knows exactly

Aha, I forgot that in Thunderbird and other email clients, one needs to click a message to see it in full and read it (right). So then Thunderbird knows exactly which messages one has read, and can highlight the unread ones in bold. Whereas here at HN, it's a lot harder for the software to know which messages one has read (because here messages are shown without the user indicating what s/he is reading). Yes perhaps different symbols — actually I'd be happy with just Thunderbird's bold font :- )