How to add thread lines in Thunderbird for Mac

If you use Mozilla Thunderbird on Mac and read messages in threaded view, it may be a little hard to track a thread tree. For instance, in this screenshot it is difficult to tell who MikeR’s last message at the bottom is in reply to.

Did he reply to his own post, Mike Easter, or perhaps a message off screen?

You can connect the messages with thread lines, using a userChrome.css tweak.

  1. In Thunderbird, click on the Help menu, then select Troubleshooting Information. This will open the about:support page.
  2. Click on Show In Finder. That will open your profile folder in the Finder.
  3. Create a new folder called “chrome”.
  4. Create a file in the chrome folder, called “userChrome.css”.
  5. Open userChrome.css, and insert this text:
    #threadTree treechildren::-moz-tree-line {
      visibility: visible !important;
    #threadTree treechildren::-moz-tree-line(selected, focus) {
      border-color: #FFFFFF !important;
  6. Save userChrome.css and restart Thunderbird. Threaded view should have lines connecting messages.