Here is how to change the fonts used in the UI for Thunderbird and Firefox.

See:

• http://www.mozilla.org/support/thunderbird/edit
• http://kb.mozillazine.org/Font_settings_in_Thunderbird
• http://kb.mozillazine.org/Pane_and_menu_fonts
• http://eriwen.com/css/tweaking-thunderbirds-chrome/

Create a userChrome.css file:

• in Thunderbird, create in ~/.thunderbird/<profile>/chrome/
• in Firefox, create in ~/.mozilla/firefox/<profile>/chrome/

You may need to create the chrome directory in Thunderbird.

Example contents are at urls above. e.g. to set the global font size:

/* set default namespace to XUL */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
/*
 * Set all the default font sizes:
 */
* {
  font-size: 11pt !important;
}