Thunderbird: change sidepane font size

In your profile directory, create or update chrome/userChrome.css (copy example file from Firefox profile dir) with the following content:

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
#folderTree > treechildren {font-size: 14px; font-family: Arial;}
#threadTree > treechildren {font-size: 14px; font-family: Arial;}
#dirTree > treechildren {font-size: 12px; font-family: Arial;}

Then restart Thunderbird!

See http://kb.mozillazine.org/Pane_and_menu_fonts