That's a GNOME or/and KDE thing. You mean that konqueror or nautilus should
start Firefox to install the xpi file.

One problem is that XPIs can be for mozilla, thunderbird, sunbird or firefox.
We simply don't know for which application the XPI is meant.
I think we can close this only as WONTFIX because of this.

Agreed, the usual way is to download from mozilla.org.  

At least in GNOME, we can associate it with all of these.

You propose we launch them all?  Or that we default to launching firefox.

Only associate them all. Nautilus now "randomly" choices one.

It's not directly related to this bug, but can fix "open with" problem in
general - instead of opening in "random" application, ask user for all MIME
types noit listed in defaults.list.

Thinking about it, I guess that it is possible to simply tweak MIME system in
following way:

- Create a simple application gnome-open-propmt. The application will be based
on Nautilus file -> Properties -> Open with.

- Associate this application with */* MIME type (It will cause, that nautílus
open with will list "Ask user to open" for all MIME types).

- Add this application to defaults.list.

Dialog will have one additional checkbox "Open files of {MIME type} every time
with this application.". It will do the same as Open with in Nautilus.