Safari comes with this functionality by default, of course. A good discussion is whether this particular function should be a core feature (Safari-style) or an extension (Firefox). It's not necessarily that whatever the user uses/wants most it should be core (for example, anti-phishing, while not demanded or often-used should be a core feature).

If you want make textareas larger, there is a bookmarklet to do it.
It adds 5 more lines each time you click it.

It is "enlarge textareas" from Jesse's Bookmarklet's site:
https://www.squarefree.com/bookmarklets/forms.html

Resizeable textarea as a User JavaScript for Opera:

http://www.webkitchen.cz/lab/opera/resizeable-textarea.js

(vertically only)