To be more precise: You need to have access to the server to be able change the files on it. Also PHP scripts normally build up the HTML output dynamically, so you won't be able to directly edit the PHP files from within the browser. The only thing that could be done is to map the URLs to the server files. Firebug doesn't allow that yet, though there's already
an issue for mapping to local development environment files.
Also note that there are some extensions like
FireFile and
cssUpdater, which allow you to synchronize at least changed CSS files.
Sebastian