How can I prevent text areas from being resizable? Text areas in Drupal 7 are resizable by default.  I would like to turn this off, but when I checked a text area with Firebug, I couldn't find the CSS that was making it resizable.

How can I prevent text areas from being resized?
It's JS that makes the text areas resizable, not CSS. I think the simplest way to disable this behaviour for all textareas is to implement THEMENAME_textarea() in your theme and remove it there:

function THEMENAME_textarea($element) {
  $element['element']['#resizable'] = false ;
  return theme_textarea($element) ;
}