This can be done with a userChrome.css file:

#threadTree treechildren::-moz-tree-row {
height: 18px !important;
min-height: 18px !important;
}

https://support.mozilla.org/en-US/questions/1310152#answer-1365539

It sounds like there is no way to change this with the settings in the program then?

I'm an old guy who doesn't know a lot about computer stuff. I don't know anything about userChrome.css files. I googled this, and found this info: https://www.userchrome.org/how-create-userchrome-css.html

If I follow that and copy the code you provided, should this resolve my issue -- and more importantly not create new ones for me?

The instructions in the link you supplied are practically the same, but more detail, as the ones in the link in my first reply. Your link applies to Firefox, but TB works the same way.

Currently, there is no way to change the settings to achieve the same result.

After some googling, I was able to stitch together information on this as most info on changing the userChrome.css file is Firefox specific and not Thunderbird specific. (Old brains work very slow, and having to reference various sites to make this work was most annoying -- Mozilla/Thunderbird team please just add in an option into the program settings to make this easier for us old people).

For anyone else that might find this thread, here is how I resolved this issue (for Windows 10) which is based mainly on the instructions here (https://www.userchrome.org/how-create-userchrome-css.html), but modified for Thunderbird:

1. In Thunderbird, open: Help / Troubleshooting Information.

2. On the Troubleshooting info page that shows up, the first section will be labelled: Application Basics. The 11th row item down is "Profile Folder". Click the "Open Folder" button.

3. Clicking the "Open Folder" button will now have opened up the folder location in Windows Explorer. The explorer path should now show that you are in the Profiles folder and it will have a path with a bunch of characters and letters as the name with .default as the extension. Click the "New Folder" option at the top menu bar and call the folder "chrome".

4. Double click the "chrome" folder and it will show a blank folder. "Right click" anywhere in this blank space and select "New" and then select "Text Document".

5. Rename the "New Text Document.txt." to "userChrome.css".

6. Open the userChrome.css file by right clicking on its name in the file list, and select "Open With" and "Notepad". (Do not use Wordpad -- if my memory is correct, Wordpad may place returns or other markers that could cause issues).

7. Paste the code that sfhowes provided in the thread above.

8. Save the file.

9. In Thunderbird, open: Tools / Options. Scroll to the bottom where you will find a button in the bottom right corner that is labelled with "Config Editor...". Click that. It will pop up an alert, click "I accept the risk". (There is only going to be a very minor change made to the config file, so don't be worried about this).

10. In the listing of config files, either type the following into the search field at the top, or scroll down to find this: toolkit.legacyUserProfileCustomizations.stylesheets

11. The value column should currently show "false". Just double click on it, and it will change to "true". This is basically a "live" setting, so there is no "save" option. Close the config editor window.

You are done.

Close and re-open Thunderbird and your line spacing will be restored. Thank you to sfhowes for providing the code needed.

Who says you can't teach an old dog new tricks :)