Pane and menu fonts

You can change the font, font size, and other attributes of fonts used in the folder pane, message-list pane, menus, and elsewhere in the user interface. To do so, add the relevant code shown below into your userChrome.css file, making changes to suit your own font preferences.

Global GUI font (except menus)

Use the following to change the font used in the folder pane, message-list pane, dialogs, toolbar buttons, etc.—everywhere except the menus.

window,
page,
dialog,
wizard,
prefwindow {
  font-family: Arial !important; 
  font-size: 10pt !important;
}

Menu font

Use the following to change the font for all menus.

menu, menulist, menuitem { 
  font-family: Times New Roman !important; 
  font-size: 12pt !important;
}

Further adjustments

There are innumerable ways you can fine-tune the fonts used in various parts of Thunderbird. For example, you can have separate font settings for the message-list pane and the folder pane, or add colors and background colors to specific elements. See these forum threads for a few examples: [1] [2] [3].