Here is the image that should have posted with the issue.

There's an add-on called 'Theme Font and Size Changer' which might help. I've never used it myself so I can't speak from experience. It's written for Firefox but is claimed to work also with Thunderbird, especially if you are using the standard default theme. It says it's especially useful for people with a visual impairment and/or a wide screen. Maybe worth a look, anyway. https://addons.mozilla.org/en-GB/thunderbird/addon/theme-font-size-changer/

I have the Theme and Font Size changer installed, but I cannot launch it with the newest thunderbird. How do I launch it inside thunderbird.

I guess that the developer of the add-on hasn't updated it to keep up with the latest release of Thunderbird, sometimes that takes a while. What operating system do you have? The reason I ask is that in mine (Ubuntu Linux) the font used in the Thunderbird menus, mail lists etc. is the same as the 'system font' set for the desktop. All programs on the computer use this unless they are specifically designed otherwise. Thus I was able recently to help another member who had the same problem as yourself: he too is a Linux user, and when he found how to change the system font he was able to get Thunderbird to his liking. Most people don't use Linux of course, but I wonder if something similar would applies in other systems. Assuming you are a Windows user, is there somewhere you can alter the default font size for your desktop? If you change that does it affect Thunderbird?

I think this add-on is broken at the moment. I have it in both Firefox and Thunderbird and in both cases the menu item it usually adds has vanished. IIRC it would be added to "Tools".

You can achieve the same result via a settings file. Have a look at Toad-Hall's suggestions here.

The system font solved the problem. I went from ubuntu-9 to ubuntu-18 and it is much easier to read. I think I will back it down to ubuntu-14 but is certainly much better. Thank you very much.