Pane and menu fonts
From MozillaZine Knowledge Base
You can change the size, color, or other attributes of fonts used in the folders pane, the message-list pane, or the menus by modifying your userChrome.css file. See the examples given in this thread (menus, folders pane, and message-list pane) and this thread (message-list pane only) and make your own adjustments as desired.
