To change the font size in the Threads Pane only, use this code in a css file:

#threadTree > treechildren {
  font-size: 12pt !important;
}

Help/More Troubleshooting, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files *.*, copy in the above code, change the number as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Preferences/General/Config. editor, restart TB.

The line spacing can be adjusted through View/Density.

To zoom all fonts, see https://support.mozilla.org/en-US/questions/1358808

Thank you. It solves the problem with the font size in the Threads pane. Now how can I change the font size/color/background in the messages (incoming and outgoing). Would like to have black background , white text color. Preferences font settings don't work now, as I am using userChrome.css file.

The text and background colors can be set in Preferences/Language & Appearance, Colors...

To change the font sizes, click the Advanced button in the same section and set the sizes and fonts with Fonts for: Latin and repeat with Fonts for: Other Writing Systems. Uncheck 'Allow messages to use other fonts'.

Thank you. After making changes in "Other writing systems" I was able to change size/background for fonts in messages. For color changes to work, I had to uncheck "use system fonts' and select 'Always' in the box "Override the colors s[ecified by the content...".

One more unresolved issue: To change the font size in the Threads Pane, I used a css file with the code you supplied. However, the background for the Threads Pane is kind of 'dark greyish', not real black. What to do to change it to real black color?

Thanks again!

Try this for the Threads Pane and Folder Pane:

#folderTree,
#threadTree
{
  background-color: black !important;
  color: cyan !important;
}

It worked for the color perfectly. BUT now all messages in all folders are in BOLD (read and unread).

Check for another part of the css for code that makes the font bold. The attached picture shows the effect of the posted css, and it doesn't bold the text. This is using the default theme.

One more question: How to change the color of unread messages to, e.g., blue?

Here is a sample you can adjust as needed:

/* Make unread messages white text and italic */ treechildren::-moz-tree-cell-text(unread) {

   color: #000000 !important;
   font-style: italic !important;
   font-weight: bold |important;  background-color: #ffffff !important;

}