Firefox should use the same default applications for opening unknown file types as GNOME, ideally through proper integration with GNOME, or at a minimum ship its own matching default settings.
Firefox's current mechanisms for determining which program to run are (a) inconsistent and (b) broken.
See also: http://bugzilla.ubuntu.com/show_bug.cgi?id=18661 and some bug I can't find right now about mad mime type handling.
This is more a case of looking at the right patches and the source than a BOF. If anyone has more useful information like that below then please provide it.
JeffWaugh: Red Hat have patches, which may be upstream in the 1.5 branch, to integrate with the xdg-mime standard (instead of mailcap). On top of that, we may want to do some mailcap/xdg-mime synchronisation, so that other apps (such as mutt) can take advantage of the xdg-mime information.
MarkShuttleworth: We don't want to see the "do you want to do this by default?" box - just DOIT! If the Red Hat patches don't land, we want to hard code this. (JeffWaugh: ... or fix mailcap, by hardcoding or synchronising.)
Data preservation and migration