If you have a general issue that fonts in UI seem too small then:

If you want to increase the font in everything then do this:

• Menu app icon > Options/Preferences > General
• Scroll down to the very bottom
• click on 'Config Editor' button to open 'about:config'
• Accept the risk.

• In search type: pixels
• Look for this line: layout.css.devPixelsPerPx
• Double left click on that line to open the 'Enter string Value' box so you can edit the Value.

The default value will be a negative number: -1.0

• Enter a new positive number eg: 1.25 or 1.5 or 2.0 or 2.2
• click on OK

the effect is immediate, so you can modify that number until it looks ok for you. Image below as guide.

re :Since font size is controlled in Firefox by CTL +/- why not the same for the main screen in Thunderbird?

Because their contruction is not the same. The code is very different. The code used to contruct websites/webpages is not the same code used to create programs that run on computers.

The zoom you mention function works to increase / decrease font size of content in the actual email.

Even if you open a program like eg: OpenOffice, the zoom only effects the contents of the document, it does not effect the program UI itself.

If you want everything you see on your computer to be of an increased pixel size then try this: In Windows 10 Start > Settings > System > Display Scale and Layout - this allows you increase/decrease general size of text. By default it is usually set to 100%. increase to eg: 150% and it increases general font size.

but if you only want to change the pixel size to effect on ly Thunderbird, then follow the instructions in my previous post.

Thank you for the information. Understand different platforms yield different features. Used your steps to fix font size. Appreciate your help.