How can I change the font size of a Thunderbird thread list on a Mac?

How do I change the font size of a Thunderbird thread list on a Mac?

I don't see any settings in the preferences.

Daniel's answer is correct. You have to extend userChrome.css. Here is the CSS code that will fix the spacing as well:

/* Set default namespace to XUL */
@namespace
url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Set font size in folder pane */
#folderTree >treechildren::-moz-tree-cell-text {
    font-size: 12pt !important;
}

/* Set font size in thread pane */
#threadTree >treechildren::-moz-tree-cell-text {
    font-size: 12pt !important;
}

/* Set height for cells in folder pane */
#folderTree >treechildren::-moz-tree-row {
   height: 25px !important;
}

/* Set height for cells in thread pane */
#threadTree >treechildren::-moz-tree-row {
   height: 25px !important;
}

You need to edit your userChrome.css file.

Go to ~/Library/Thunderbird/Profiles/[yourProfileName]/chrome/ (creating the chrome directory if it doesn't exist) and edit userChrome.css, again creating if it doesn't exist.

For example, to edit all text in Thunderbird (except message view), enter the following:

/* Global UI font */
*
{
    font-size: 18pt !important;
    font-family: Verdana !important;
}

The result looks something like this:

Unfortunately, I was unable to find the CSS class for the message list. Replace the * in the CSS content with that CSS class, and it should work as intended.