Do you use any custom userChrome.css file?

Yes I do...

/* Set up the toolbars so the menu bar is on top and the tabs bar is second

  all above the new unified toolbar. I hide the menu bar and enable the Windows
  title-bar so everything looks like TB v102. */

1. tabs-toolbar {

 order: -1;              

}

1. toolbar-menubar {

 order: -1;             

}

/*Thunderbird now offers View > Density > 'Compact', 'Default', 'Relaxed' options */ /*#threadTree > table[is="tree-view-table"] td {

 line-height: 21px !important;

} */

/* table view column buttons */

1. threadTree button {

 background-color: #e0ebf5 !important;
 font-weight: bold !important;

}

/* message list hover color */

1. threadTree button:hover {

 background-color: #cbe4fb !important;
 font-weight: bold !important;

}

/* This next section is experimental to try to change the hover characteristics

  of the folder pane section */
  

/* Folder pane hover */ /* .container:hover {

 background-color: #a8d1dc !important;

}

li.selected > .container:hover {

 background-color: #a8d1dc !important;

}

1. folderTree:focus-within li.selected > .container,
2. folderTree li.drop-target > .container {

 background-color: #a8d1dc !important;

}

• /

/* This is the end of the experimental section. It works but the default selection

  when first opened doesn't use the new color(s). */

/* message list threads row height */ tr[is="thread-row"] { line-height: 24px !important; }

/* folder pane row height */ li[is="folder-tree-row"] { line-height: 23px !important; }

/* threads pane card height font */ .thread-card-container{

    height: 60px !important;
    font-size: 18px !important;

}

I hope this isn't the culprit. I'd never have upgraded TB unless the interface was fixable with this.