Built into Firefox is "Scratchpad" (an on the fly JS editor). The Scratchpad window is an implementation of CodeMirror. The code itself is utilized across many of the Firefox Dev Tools. Within the Firefox Dev Tools is a "Style Editor". Everything you need to access|change a site's CSS and custom User Css is implemented by Firefox except none of it is exposed, and there is no management gui to do so.
So we need to use Stylish or the mostly-broken-for-the-last-year "User Style Manager". Neither of these addons implement CodeMirror|scratchpad. USM's editor is the thing that breaks constantly and poorly implements some of the features of a Scratchpad window. Neither of these addons allow you to use a custom (external) editor for css - like GreaseMonkey does. Stylish stores your CSS in database files, so when Stylish breaks you don't even have css text files that you can access.
There's many such features like this in Firefox
Just about every computer on the market today runs Unix, except the Mac (and nobody cares about it). -- Bill Joy 6/21/85