<p>For example if I put "C:\docs" in address bar of Firefox, it shows all files in the directory. Is it possible to customize this page with CSS?</p>

<p>Apache 2.2 allows us to do it using IndexStyleSheet directive, so I wondered if Firefox can do it.</p>

Firefox has a file userContent.css for each profile (found in the profile's settings folder). That file defines the standard CSS, why may then be overwritten by a website. So they also apply to directory listings.
You should have a look at the actual HTML code Firefox is creating for the directory listing to see how to do the CSS definitions right. You will find extensive documentation about userContent.css on the web.
http://www.mozilla.org/unix/customizing.html#userContent