In Firefox 27, Codemirror is now deployed as the default code editor:

Codemirror

Codemirror is a popular HTML5-based code editor component used on web sites. It is customizable and theme-able. The Firefox Devtools now use CodeMirror in various places: Style editor, Debugger, Inspector (Edit as HTML) and Scratchpad.

From the Option panel, the user can select which theme to use (dark or light).

Source: Firefox Developer Tools: Episode 27 – Edit as HTML, Codemirror & more

This works for me in ~/.mozilla/firefox/profile-name/chrome/userChrome.css.

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document regexp("chrome://browser/content/devtools/.*"){

* { font-size: 20px !important }

}