The font for the Folder and Threads Panes can be set with the following css:
.tree-rows
{
font-size: 12pt !important;
}
Help/Troubleshooting, Profile Folder, Open Folder, close TB, create a new folder named chrome, create a new document in chrome with a text editor, name it userChrome.css, Save as type: All files ., copy in the above code, change the number as desired. Double-click toolkit.legacyUserProfileCustomizations.stylesheets to true in Options/General/Config. editor, restart TB.
The font for the menus and submenus can also be adjusted with css, if that’s what you mean.
Let me try to give some more detailed instructions:
First, go to “Help” → “Troubleshooting Information”, find the entry “Profile Folder”, click the button “Open Folder” next to “Profile Folder”. It will open the profile folder.
Now you should see some folders like “crashes”, “datareporting”, “minidumps”, “safebrowsing” etc.
(By the way, it is a good idea to make a safety copy of the complete profile folder.)
Create a new folder named “chrome”, so that it appears alongside with the folders named above, i.e. it appears in the same folder as the folders named above.
Inside the chrome folder, create (e.g. with notepad.exe) the file “userChrome.css” with the content:
(Please make sure that the file is named “userChrome.css” and not “userChrome.css.txt”. Maybe you should google "how to make windows show file extensions.)
Next, in Thunderbird you need to open “Preferences” → “General” → “Config Editor…”, accept the warning dialog.
Search for the entry “toolkit.legacyUserProfileCustomizations.stylesheets” and double click it to set its value to true.
This add-on adjusts tree children font sizes from 12px (default size) to 16px.
Also, this add-on adjusts tree children heights from 13px(?) to 50px.^^
Also, this add-on has option to dot cells.
Usage is after clicking menu button, please, move mouse pointer to tree,
if so, heights are changed at hovering or clicking a tree child.