How do I get an application to appear as a choice in update-alternatives?

I separately installed the Firefox Beta and Alpha channels, and have desktop configuration files pointing to them in ~/.local/share/applications. However, stable Firefox is being used as my default browser by the system. (Firefox Beta used to be used until I messed with the "Default Applications" in System Settings, where it is not listed.)

I tried running sudo update-alternatives --config x-www-browser to manually change it, but it's only recognizing Chromium and Firefox (stable) and showing them as a choice.

What can I do to get custom desktop configuration files in ~/.local/share/applications to be seen as default alternatives? I think I may have to fiddle with the desktop config files, or with mimeinfo.cache or mimeapps.list?

Running Oneiric.

Here is the content of the firefox-beta.desktop file I created:

[Desktop Entry]
Name=Firefox Beta
Exec=firefox-beta -P Beta -no-remote
Icon=firefox
Terminal=false
X-MultipleArgs=false
Type=Application
StartupNotify=true
StartupWMClass=Firefox
Categories=GNOME;GTK;Network;WebBrowser;
Comment[en_US]=Firefox Beta Channel
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/vnd.mozilla.xul+xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;
Name[en_US]=Firefox Beta

[NewWindow Shortcut Group]
Name=Open a New Window
Exec=firefox-beta -new-window about:blank
TargetEnvironment=Unity

You're looking for the this command:

sudo update-alternatives --install /usr/bin/x-www-browser x-www-browser /path/to/firefox-beta 100

That adds /path/to/firefox-beta to the x-www-browser alternatives with a priority of 100 (auto-switching to it unless there's something else with a higher priority). You can then use update-alternatives --config as usual. To remove it, run the following:

sudo update-alternatives --remove x-www-browser /path/to/firefox-beta

This removes that particular entry from the alternatives for x-www-browser.