Well, it is a "new mail" marker. I think that means it indicates "stuff that's arrived here since the last time you checked this folder", as opposed to "Unread", which is indicated by bold typeface. So I think it's doing what it's supposed to do.

How does being in bold not suit your needs?

Do you use the Star label for any particular purpose? Could you use that in place of the "orange cog"? (I always thought of the cog as an asterisk.)

How do you move messages to other folders? I'm thinking that if you use filters, the filter may be able to set the Starred status when moving the message. Or you could use a catch-all incoming filter that places a star on each and every message as it arrives, so that will stay on each message until you manually clear it.

Adding tags is another option; it's very simple to clear the tag on a message - just select it and type 0. Tags can colour a message and you can add a Tags column to the Thread List display if you need to be able to read the tag names.

Thank you for your quick reply. The operation of my Thunderbird has changed from a few days ago. Ver 38.7.0 now. I do use tags. My apologies for not giving enough info in the first description.

What is different now is the the folder title is no longer blue after I've read only one of several new messages in the folder (presumably because looking at one new message removes the 'orange cog" from all other new messages in that folder). I no longer see in which subfolder new messages exist that I've not looked at. I have about 25 folders in one inbox and about the same number in the other major inbox (different email account). A few weeks ago I changed the filters to separate incoming mail into more categories (more Inbox subfolders). This performed as expected. It's only lately that the blue in the folder title changed back to black after reading reading a single message. So I lose track of new messages.

In my Thunderbird, the blue mail account name in Folder Pane has always only been blue if I have received new messages and not accessed the inbox of that account to view new mail. Once I have accessed the Inbox, regardless of whether I have read emails or not, the blue will change to black. The blue coloured font means, there are new emails in this account since the last time you accessed that mail account folders. As far as I can remember, it has never meant you have 'old unread mail'.

Any folder with unread mail will remain in bold font, so drawing your attention to it. Bold font means any unread mail, new or old.

But in your case, those folders might be collapsed - not immediately obvious where all the unread messages are now lurking.

However, your situation of locating various unread mail in possibly various folders can be resolved a different way.

To show only the folders in the Folder Pane that contain unread mail: 'View' > 'Folders' > 'Unread' or via Mail toolbar: 'Menu icon' > 'Folders' > 'Unread'

Selecting: 'View' > 'Folders' > 'All' will get you back to showing all your folders again. If you have Inbox with no unread emails and it has a subfolder with unread emails, this method will show the Inbox and only the subfolder with unread mail. All other subfolders with no unread mail will disappear from the view. This makes it so much easier to locate all unread mail.

To show only unread mail in the folder you are currently viewing: click on the 'Show only unread messages' icon in the 'Quick Filter Bar'; it looks like a pair of sunglasses. click on same sunglasses icon again to go back to showing all emails.

Whilst in a folder, if you use 'Threading', then it is also possible to miss emails in collapsed threads. 'View' > 'Threads' has various options to only show eg: 'Threads with unread'.

'View' > 'Threads' > 'All' will take it back to default settings.

Using these methods to display exactly what you need should help your workflow and make life much easier locating those unread emails.