CSS Live Reloader is a Firefox extension to ease website CSS development. The extension reloads a page CSS whenever it changes on the server, resulting in something close to live editing of the stylesheet.
So how does this add-on work? Here is more information about it from the developers:
To detect changes to the CSS, the extension relies on a WebSockets daemon running on the same webserver the page originated from. The daemon pushes down notifications whenever it detects changes to the CSS. Even without a notifier running, the extension behaves normally, albeit without the live component. You may press F9 to request a refresh of the current page CSS. Simple usage requires nothing more than the installation of the extension. Press F9 whenever you wish to reload the page CSS.
Want to pick up this amazing add-on? You can pick up the CSS Live Reloader from the Firefox Add-on website.