* Do not remove the @namespace line -- it's required for correct functioning
*/

* Make all the default font sizes 9 pt:
*/

* Make all the default font sizes 9 pt:
*/

   font-size: 9pt !important; 
   font-family: Arial !important; 

Just this, y'all. If you found 11059537, archived, and the path in there didn't solve your problem, I found that the functional dir for my *.CSS files is /Users/{LOGON_NAME}/Library/Thunderbird/Profiles/tre3jyrg.default-release/Mail/Local Folders in my install dated 2019-11-19. The coding in the css solution by Zenos 1059537 works perfectly when I put the CSS file in this dir. QUOTE (sorry, I dunno how to insert indent coding in this box): I have to use the userChrome.css file for both Thunderbird and Firefox on my Linux boxes, since support for this add-on in Linux was dropped some time ago. Create a folder in your Firefox profile named chrome In this folder, create a text file named userChrome.css In that file place the following code: /* * Do not remove the @namespace line -- it's required for correct functioning */ @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */ /* * Make all the default font sizes 9 pt: */ * { font-size: 9pt !important; } Obviously, edit that font-size: 9pt !important; to a value that suits you. I generally use the same point size as was selected in the OS's desktop settings. You can also set the font face here, e.g. /* * Make all the default font sizes 9 pt: */ * { font-size: 9pt !important; font-family: Arial !important; } A similar file named userContent.css in the same chrome folder can be used to set the size of the content of webpages (and of email if used with Thunderbird.) END QUOTE So wherever your installer dropped Thunderbird is where you need to create "userChrome.css" as an ASCII and insert Zenos' code. Thanks, Zenos. Worked like a charm, and enabled my old eyes to actually see the incoming mail! Otherwise, Thunderbird, out!