Aha! Ted and Dom showed me how to add a tel: URI scheme handler to firefox. It turns out that firefox consults the gnome configuration database, so this works:

$ gconftool-2 -t string -s /desktop/gnome/url-handlers/tel/command "echo %s"

In place of echo, I use a little script that Ted wrote that uses curl to drive Vonage Third Party Call Control. This works great with the hack to build an hCard version of my sidekick contacts!

tags: UriSchemes, sysadmin, hCard.