Description of problem:

One of the test cases in the GNOME3 test day is:
https://fedoraproject.org/wiki/QA:Testcase_desktop_uri, which says:

"Clicking on a mailto: URI should result in an appropriate email client running and creating an empty email message to the appropriate address"

The problem is that Firefox doesn't respect the desktop's default URI handlers in GNOME 3.
Firefox uses its own system for specifying which applications open which URIs (e.g. mailto: URI). The first time the user clicks on an URI, it prompts the user which application should handle the URI (it offers GMail and Yahoo Mail and doesn't offer Evolution (which is set as the default email client in my GNOME desktop).
The user has the option of manually specifying the application by browsing to its location (e.g. /usr/bin/evolution), but that's not an elegant way of selecting Evulution as the URI handler in Firefox.

Version-Release number of selected component (if applicable):
firefox-4.0-3.fc15.x86_64
evolution-2.91.92-2.fc15.x86_64
gnome-shell-3.0.0.2-2.fc15.x86_64

How reproducible:
Always.

Steps to Reproduce:
1. Run the default web browser (Firefox) for the desktop, and browse to a page with a mailto: URI, such as this one: http://fedoraproject.org/en/contact
2. Click on the link (in the example page, click on the address press)
3. Wait for the result
  
Actual results:
Firefox chooses the user, which application should handle the mailto: URI.

Expected results:
Firefox should open up Evolution, the default email client as set by GNOME3's System settings -> System Info -> Default Applications.