OK, so for dunces like me on mac, I have pulled together from a few threads what worked for me (I use a mac OS10.10.5 but I don’t think the mac OS will make a difference?) in case anyone else on mac stumbles on this thread.
(All I wanted to do was increase the font size in the email list pane of the inbox so that i can visually scan the list for important emails.)
In Thunderbird, go to Preferences/Advanced/General/Config. editor
Accept the responsibility for armageddon when the pop-up asks you (but the pane needs to be sized large enough to allow you to see the continue button in the popup, which it wasn’t the first couple of times I tried. For some reason it wont load the button in the popup if the pane is too small so you can’t scroll down to it unless you click when the pane is already larger)
Scroll down the list to layout.css.devPixelsPerPx
Double click that line to open the editor popup and change from the default (-1.0) to 1.1 (but others in different posts have preferred 1.2, 1.25, 2 and even 4. Start at 1.0 or 1.1 if you are worried it might be too big, and that is with no negative sign.)