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.

1 Response

  1. Tony Mechelynck October 29, 2011 / 9:08 am

    Not only Thunderbird, and not only on the Mac: I’m using SeaMonkey on Linux64, and thread lines aren’t displayed either by default. So I added the exact same CSS rules in (for SeaMonkey) $HOME/.mozilla/seamonkey/????????.default/chrome/userChrome.css, restarted, and voilà! 🙂

    You may be interested in the following rule (which I have just below yours) to outline the current line in the thread pane, even when there is no “selected” line:

    * Force a highlight on the current line of the Mail/News window’s
    * List pane (also the folder tree pane)
    { border: 1px dotted green !important

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