Finally got frustrated that despite the obvious settings, Thunderbird was not using Firefox when I click on a link in an email on my Debian desktop.  Instead it chose to launch Epiphany, despite the fact that I\'d set Firefox as my browser under Gnome -> Preferences -> Preferred Applications.  If you\'re running into the same issue, as root run

dpkg-reconfigure thunderbird

And select GNOME as the preferred way of browser integration.