rsteinberger said

Why in the world the Mozilla developers haven't added a simple config setting to tweak this is beyond me. Can anyone help?

Why apple would do something so different that a special tweak would be required for their products is beyond me. SO we are both left wondering you about Thunderbird and me about Apple.

More to the point have you actually changed the scaling using the add-ons options. The following is from the download page here https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer-for-tb/

THUNDERBIRD USERS: Statusbar Bar Button: Click on the Theme Font & Size Changer button inside your Thunderbird status-bar. Tools Option: Choose Tools menu from Thunderbird Menu Bar and click on the Theme Font & Size Changer option. It will open up Theme Font & Size Changer settings panel. If you have any support questions, feature requests and bug reports, please do not hesitate to contact us by below support email address: support at barisderin dot com Regards, Baris Derin

Blaming Apple for Mozilla software "features" isn't going to win you any supporters.

I have installed the Theme Font & Size Changer option. I go to the Preferences but don't see any setting for font sizes. IOW, it doesn't appear to do anything at all.

Did you install the fixed version?

https://addons.mozilla.org/en-us/thunderbird/addon/theme-font-size-changer-fixed/

You should see settings for the add-on under the Tools menu and a button next to the AppMenu button.

Seems to have worked. This extension should be included with all releases of Thunderbird and integrated into the Preferences.

Thanks.

On a macbook pro running High Sierra, I installed Thunderbird 60.0 (which it oddly took me a while to find, seeing mostly older versions to download, and my previously installed 52.9.1 shown as "up to date"), as well as the "fixed" version of the "Theme Font & Size Changer" extension, said to work with TB versions 58.0 - 65.0 (if there's a version newer than 60.0, why dont I see it on the TB download page???). However, there is no associated button in either the Status Bar or in the Tools options as stated, and as far as I can tell, the extension did nothing. Im seeking to increase the font size of the email listing display.