I installed the latest version of Comix ( http://www.gnomefiles.org/app.php?soft_id=1055 ) and since then, Firefox is insisting on opening all .tar.gz files with Comix instead of Gnome's archive manager (as the default should be). Of course, I emailed the author and reported the bug to him, but he insists that the bug is either in Firefox or in Gnome and not in his application. I copy below his reply:

*******
Something has changed, the desktop file (comix.desktop) now includes
information about the supported mime types (application/x-gzip among others).

The thing is that the desktop files (which conforms to a freedesktop.org
standard -
http://www.freedesktop.org/wiki/Standards_2fdesktop_2dentry_2dspec) does
not say anything about which applications should be the default for
different mime types. To quote a portion of the standard:

"The MimeType key is used to indicate the MIME Types that an
application knows how to handle. Applications that can handle multiple
MIME Types would list all of the ones it can handle in a ';' separated
list, as normal. It is expected that for some applications this list
could become long. An application is expected to be able to reasonably
open files of these types using the command listed in the Exec keyword.

There should be no priority for MIME Types in this field, or any form of
priority in the desktop file. Priority for applications is handled
external to the .desktop files."

So how to use this data, how to choose a default application and how to let
the user change the choices is up to the specific program (e.g. Nautilus,
Firefox, etc.) that wants to open the file with another application (e.g.
Archive Manager, Comix, etc.). So unless I am much mistaken, if Firefox
does not let you change it's default choice (which happened to be Comix for
some reason), it's rather a bug in Firefox than anything else.

Regards,
Pontus
*********

So, *if* that's not his bug, then it's a bug in Arch Linux as far as I am concerned.