yes the same option is on the mac... in preference not options. but it will not help as it is set to immediately unless you already changed it.

Try restarting with add-ons disabled from the help menu and see if that has an effect.

Thanks Matt. By 'option' I wasn't referring to to a tab but just saying that the text/tab that was supposed to be there, the option, wasn't there. so I still don't know how to set the 'delay' before a message is marked as 'Read' Restart with add-ons disabled. I have just done that but messages are still staying bold after opening. I opted for the 'safe mode' of restart with add-ons disabled - is that right? I've never consciously done anything in 'add-ons' or changed anything else. Would another option be to remove Thunderbird and then download it again but would that delete all my existing emails?

In Windows, it is under Tools|Options|Display|Advanced

As Matt has pointed out, in a Mac that will be a little different.

http://kb.mozillazine.org/Menu_differences_in_Windows,_Linux,_and_Mac

…tells us it's under Thunderbird|Preferences|Display|Advanced if you go by the old fashioned main menu across the top.

Note that under "Application button" it fails to point out that most of the navigation is done by hovering, and not by clicking on things.

Uninstalling/reinstalling is almost always entirely futile because by design doing this doesn't touch your stored messages, nor your settings. So whatever setting is awry at the moment will remain as it is now.

Thank you, Zenos & Matt. Your ' Thunderbird|Preferences|Display|Advanced' enable me to find it after all. I was selecting Advanced from the top line of options instead of the 'Advanced' within 'Display. You're stars!

Sorry but still a problem. I selected 'automatically mark messages as read' - 'immediately on display' which worked fine in that the subject line on the list of emails went from bold to medium but then, having closed the Thunderbird window and reopened it only 'unread' messages were listed. How can I set it to show 'all messages'? please. I have looked but there is only the little icon to ' show only unread', no icon to 'show all messages'

Ah, curiouser and couriouser. Have just scrolled down the 'messages' window and at the top are all enread messages starting now and going back to 20th May. below there are all the messages I have opened - ones in medium type. Why? I wonder. So the read messages are there, just not intermingled in date order as they used to be. How do I stop 'unread' messages taking preference? please

Ah, I've clicked on the head of the 'Date' column and now 'read' and 'unread' are in date order so hopefully that will be the end of the mystery and I can carry on as usual. Thanks again