Green is the default. Perhaps you had an addon or theme installed that showed that column as blue?

I made an 8x8 image, unreadmail.png, and stored it in the chrome folder along with this userChrome.css file:

treechildren::-moz-tree-image(unreadButtonColHeader, unread) {
  list-style-image: url("unreadmail.png") !important;
  -moz-image-region: rect(0px, 8px, 8px, 0px) !important;
}

See attached picture. Reference.

Set toolkit.legacyUserProfileCustomizations.stylesheets to true in Config. editor.

Wayne, maybe green is now the default, but it was not the default for the past 10 years or more I have been using Thunderbird up until yesterday's update. I started using Thunderbird when Mulberry died. I have never installed any special themes.

Sfhowes thanks for the hack. Where is the chrome folder in OS X? I do not see it in Library/Application Support/Mozilla, Library/Thunderbird, Library/Preferences or Applications/Thunderbird ==> Show Package Contents

Thanks -- Dan

Help/Troubleshooting, Profile Folder, click Show in Finder, close TB, create a subfolder of the profile named chrome, and in chrome create a new document with a text editor, save it as userChrome.css, copy in the code above, and copy an unreadmail.png file into chrome. Make sure the css file type is Cascading Style Sheet, not Text. I've attached an 80x80 png file that you can resize or change the color.