First, you need to specify that external modifications via CSS are to be allowed. To do that, change toolkit.legacyUserProfileCustomizations.stylesheets to true in config editor.

Second, create a folder named chrome in your profile.

Third, create a text file named userChrome.css with this content:

See later post. The CSS was not being properly formatted here.

First, you need to specify that external modifications via CSS are to be allowed. To do that, change toolkit.legacyUserProfileCustomizations.stylesheets to true in config editor.

Second, create a folder named chrome in your profile.

Third, create a text file named userChrome.css with this content: (see attached image)

(change font size to your preference. To see changes, TB must be closed and restarted.)

Thanks for the code. BTW I used SnagIt screen capture to capture the text in the image you provided. It made it easier to put the text in my userChrome file  : )

I tried both pt and px. I like px. The line where it says -- threadTree treechildren: :-moz-tree-column had a space between the : : and it would not work till I used  ::

The code I had used before is here: http://davidswebsite.com/Firefox_tabs/firefox_tabs.html The other css code you see adds lines to separate the messages.

Question. How to change the display font for either?

This is a way to add the font you want. You can also use the code above from 4:16am and add in the line for the font. You can change the color and size and the font. Making the text Bold is not working so far.... I did try font-weight: bold; See attached image for the css.

I tried the following in the userChrome.css file.

"Open Sans" "Trebuchet MS" "Times New Roman" "Bookman Old Style" Verdana "Segoe UI" Calibri "Noto Sans" Arial

To change the font-family for the Threads or Folder Pane, a setting in Preferences has to be enabled:

https://support.mozilla.org/en-US/questions/1360794#answer-1465754

Here is a fix called UI scaling: Adjust font sizes for window that has Subject - Correspondents etc and also the left sidebar that has the Inbox - Sent - etc. This basically zooms in. Message list Folder list

Thunderbird> Preferences> Config Editor Open the config editor. Click on Show All. Search for: layout.css.devPixelsPerPx You will see -1.0

Change to 1.2 as an example. Once you click on he save 'check' button it will change right away. I have mine at 1.1 I am using a 32" monitor and use 2560 x 1440 screen resolution and the text size looks good now.

If too big or small just change till you like what you see.