That's because the top thread is not unread.

I have one folder which is always set up as threaded. I know it has a new message because the folder is shown as bold.

I use the 'Quick Filter Bar' . Click on the 'Show only unread messages' Click on same icon again to go back to normal threaded view.

Yes, the top thread is not unread. I came from Postbox, they would bold the top thread as well, it is more user friendly

Yes, it would be nice if this could be customized. Currently the top message is only underlined when a thread is collapsed, which makes it much less visible compared to bold font. I missed several important emails because of this.

Could there be a config editor option please?

Ok I've got some info for those who want collapsed threads to show the primary email in bold instead of underlined, until expanded.

In Thunderbird:

• Help > Troubleshooting Information
• click on 'Open Folder' button

a new window opens showing your Profile folder.

• Close Thunderbird now - this is important.

• Create new folder and call it chrome note the spelling - use a lower case 'c' It should be in the same place as the 'Mail' folder.

• open Notepad:
• Copy all the text between lines below and paste it into the Notepad document.

/*
* Do not remove the @namespace line -- it's required for correct functioning
 */

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); 

/* set default namespace to XUL */

/*
    * Collapsed read thread, but the thread contains unread , set text to bold black 
*/
   treechildren::-moz-tree-cell-text(container, closed, hasUnread, read) {
     text-decoration: none !important;
     font-weight:  bold !important;
     color: #000000 !important;
   }

/*
    * If thread selected, revert to highlight text color
*/
   treechildren::-moz-tree-cell-text(container, closed, hasUnread, read, focus, selected),
   treechildren::-moz-tree-cell-text(container, hasUnread, read, focus, selected) {
     color: HighlightText !important;
   }

• Save the document as filename userChrome.css in the chrome folder. Note the spelling all lower case except for the 'C'.

Double check all the copied code is correct (view info in this forum questions) as sometimes the !important in code part seems to get missed by some people, so not sure if something is being altered in the emails you receive. Edit and resave if necessary.

• Restart Thunderbird.

Thanks Toad, I have done the settings, but it still does not bold the top

Please check the userChrome.css file.

I notice that there is a /* missing right at the start.

It should be:

/* 
* Do not remove the @namespace line -- it's required for correct functioning
 */

But you seem to have:

the first /* is missing
* Do not remove the @namespace line -- it's required for correct functioning
 */

• Exit Thunderbird
• Access userChrome.csss file
• Add the /* at the start
• Save File
• Start Thunderbird.

Oh, you are correct. I miss that! It works fine now, thank you very much!

Good to hear all is now ok for you.