AFAIK, what's happening is that Drupal is adding the ?param onto the CSS file name when it includes it in the HTML, to prevent your browser caching it.
So it's not the FB debugger, it's the Drupal framework. FB is just showing how the CSS file name looks when Drupal incorporates the file.
The actual file name itself is unaffected; it's just the way it's shown in the relevant <link> tag in the <head> of the generated HTML.
Adding an artificial 'parameter' onto a file name like is a common trick used to prevent browser caching.
So you can edit the CSS file, which usually resides in the theme directory, using it's normal file name.
If you want more info on this, try the
drupal.org forums. What you really here is a Drupal question :-)
HTH
Alan