There are a couple of issues here:
1) Firefox does not absolutely know that it is pulling from a static
file. All that it knows is that it is reading the response from the
server and the server can map any URL to any response. One of my own
projects uses a servlet to generate CSS for the purposes of
aggregation and cache control.
2) The information that we have available to Firebug is not enough to
properly regenerate the complete CSS file. We have to make assumptions
about things like formatting and we have absolutely no information
about comments that existed in the original source. Depending on the
goal of the author this might be sufficient, if they are just looking
for something that is machine readable. If they are looking for
something that is more human readable, they may have to make some
additional changes to this source to tweak it.
Firediff will hopefully have the export to local file system
capability built into the 1.0 release, which should hopefully occur
sometime around the release of Firebug 1.5.
http://www.incaseofstairs.com/
should have an announcement posted when this release is made
available.