Comment 2 for bug 670128

Hmm. I think this is related to

firefox (4.0~b7+nobinonly-0ubuntu1) natty; urgency=low
  …
  * Add x-scheme-handler magic to desktop files
    - update debian/firefox.desktop.in
    - update debian/firefox-beta.desktop.in
    - update debian/firefox-nightly.desktop.in
    - update debian/firefox-unofficial.desktop.in
  …
 -- Chris Coulson <email address hidden> Wed, 17 Nov 2010 21:35:57 +0000

which causes these warnings when running ‘update-desktop-database’:

$ sudo update-desktop-database
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/http" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/https" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/ftp" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)
Warning in file "/usr/share/applications/firefox.desktop": usage of MIME type "x-scheme-handler/chrome" is discouraged (the use of "x-scheme-handler" as media type is strongly discouraged in favor of a subtype of the "application" media type)

If I remove the x-scheme-handler types from /usr/share/applications/firefox.desktop and rerun ‘update-desktop-database’, the problem goes away, and gnome-open lets me use my preferred web browser again.