Firefox has scratchpad built-in for editing JS, HTML or CSS.
Other Extensions, like Greasemonkey give an option to choose what editor you wish to use.
User-Style-Manager would be nearly perfect if it allowed the end-user to choose an alternative editor for CSS (or even ScratchPad). USM would also (likely) stop breaking every time * Firefox is updated.
(*) USM hasn't worked properly in Nightly, or FF Dev since version ~42.