Strangely, right before the subject lines of the unread messages were blue, but now they are not no more. The same actually happened yesterday, for a short while there was a change but it only lasted short time. So its a very unstable result I got. BTW, the script that I use is this one (and as far as I understand it should both increase the font and color it:

/* Increase the font size of message titles */

1. threadTree > treechildren::-moz-tree-cell-text {

   font-size: 16px !important; /* Adjust the size as needed */

}

/* Differentiate unread messages with background color */

1. threadTree > treechildren::-moz-tree-cell-text(unread) {

   font-weight: bold !important;
   color: blue !important; /* Blue color for unread messages */
   background-color: #e0f7fa !important; /* Light cyan background for unread messages */

}

/* Differentiate read messages */

1. threadTree > treechildren::-moz-tree-cell-text(read) {

   font-weight: normal !important;
   color: black !important; /* Black color for read messages */
   background-color: #ffffff !important; /* White background for read messages */

}

/* Ensure manually marked unread messages also appear with the background color */

1. threadTree > treechildren::-moz-tree-cell-text[unread] {

   font-weight: bold !important;
   color: blue !important; /* Blue color for manually marked unread messages */
   background-color: #e0f7fa !important; /* Light cyan background for manually marked unread messages */

}

Try completely disabling your userchrome and see if it is still unstable. Userchrome is really not mean to general users and usually requires some sort of tweaking for every single release.

While some will promote userchrome here, I do not as it is not supported by the developers and therefore is another support issue in the wings waiting. https://support.mozilla.org/en-US/kb/userchromecss-js-usercontent-unsupported

Thanks you for clearing that up. I have deleted userchrome now. It is stable now, in the sense that now I never get the desired blue or increased font for unread messages. Is there a valid approach to make new messages stand out?

Edit: Actually, it makes new messages blue, - but they don't stay blue; if one closes TB and opens it again, they are all black, read or unread.

Are there really no solution to this problem?

It is especially an issue when friends send emails without a subject line. The subject line is the only line that TB makes bold when unread, resulting in NO distinction between read and unread messages in emails lacking a subject line. It would be a great improvement if the from-line could also be in bold font while the email remains unread.