https://support.mozilla.org/en-US/questions/1270950

it did not work for me

re : the FONT SIZE add-on doesn't work This implies you have something effecting things. Have you uninstalled the 'Font Size' addon that does not work? If no, uninstall addons that can effect the font size OR Start Thunderbird in Safe Mode to block it so you can test.

In Thunderbird (not Firefox browser) 'Menu icon (2 lines)' > Options > Options > 'Advanced' > 'General' tab click on 'Config Editor'

In top search type: Pixels look for this line: layout.css.devPixelsPerPx; By default it should have a Value of -1.0 Double click on that line to open a small window. Clear the content and type: 1.5 click on OK

Either I am missing something or just not understanding how to get to where I can edit the config variables. Being sight impaired the small display font is very difficult. I also can confirm that it is not just my PC. My wifes one with no add-ons has the same problem.

lordjohn said

Either I am missing something or just not understanding how to get to where I can edit the config variables. Being sight impaired the small display font is very difficult. I also can confirm that it is not just my PC. My wifes one with no add-ons has the same problem.

Accessing the config. editor is described in the previous post. Another route is Tools/Options(Mac:Preferences)/Advanced/General/Config. editor. Press Alt if the Menu Bar with Tools is hidden. Once you open Config. editor (ignore the disclaimer warning), follow the previous post to adjust the preference.

Ah the Config box was off the right of my window - found it and much better now. Will adjust to taste. Thanks.

One more question - what is the difference between having the vakue positive eg 1l5 and negative eg -1,5  ? Thanks

It seems that any negative value has no effect. The effect of any positive value is apparent as soon as the pref. is changed.

I can't help with these problems. Having logged in I can't get back to the discussion about fonts in Thunderbird. The posting seems elusive.