Dear Maintainer,
This incapability is two-fold. The value for the default for "Calendar"
from gnome-control-center is not available for Iceowl, nor will Iceowl be
opend from GNOME's panel's "Open Calendar". This lack of functionality
inhibits Iceowl from being used like most other programs for calendars. I
can't remember the functionality of GNOME2 and the default for "Calendar".
Ubuntu has the same issue or similar. [1] They're seem to be work-arounds.
[2][3] Perhaps iceweasel-extension needs an iceweasel.desktop which would
be a copy of icedove.desktop with a mime-type for calendar, or just adding
the mime-type for icedove.desktop within the installation of Icedove. [4]

[1] - https://bugs.launchpad.net/ubuntu/+source/gtk+3.0/+bug/841409
[2] - http://forums.fedoraforum.org/showthread.php?t=270376
[3] -
http://unix.stackexchange.com/questions/41670/how-can-i-make-the-gnome3-calendar-use-thunderbird-instead-of-evolution
[4] -
https://bugs.launchpad.net/ubuntu/+source/gtk+3.0/+bug/841409/comments/5

EXTRANEOUS LINKS
[1] - https://bugzilla.redhat.com/show_bug.cgi?id=689288
[2] - https://addons.mozilla.org/en-US/thunderbird/addon/evolution-mirror/
[3] - http://forums.linuxmint.com/viewtopic.php?f=151&t=85318