text/x-csrc, inline instead of popping up that download box? (You could imagine always showing it as text and then letting the user pick "Save As" or you could add a widget to the download box saying "show me the damn thing" -- I don't really care about the interface too much. You'd think an extension could do it, too.)
The answer, as with seemingly every annoyance I have with Firefox, is that: it's a known bug; the bug is over five years old; it's been re-reported a million times and had each marked as a dup; it involves a side argument about whether to follow an RFC or whether to do "the right thing", aka what every user expects it to do, and break specs; it also has a multitude of side arguments about whether a proposed UI is too complicated or not, all by a bunch of people who feel that adding their opinion somehow helps the discussion; it's ballooned into a bunch of different features, all of which are much less importantant than the core idea but make it that much more difficult to implement; and it's apparently incredibly difficult to fix, blocking on other bugs that don't have owners because they're too hairy. Or, to phrase it differently, it looks like a totally obvious feature but will never happen.
(I guess at least I'm better off than jwz, for whom these sorts of bugs just get closed WONTFIX. I still have a tiny bit of hope.)