Firebug doesn't support source maps yet (as of 2.0.*).
You may want to follow issue 5765 for source maps support for JavaScript and issue 5961 for CSS.
Firebug 3 will integrate into the built-in DevTools of Firefox, which already have this feature. So Firebug 3 will also have that feature. Development versions are available at getfirebug.com.
Update:
Firebug is merged into the Firefox DevTools since Firefox 48.