Both on my Debian PC’s at home and work, I’ve been annoyed that my Firefox and Thunderbird installations didn’t launch each other when I clicked on a link or email address. For example, at work, Evolution would come up if I clicked on a mailto link on a web page. Inside each program’s preferences there were no options for controlling this behavior. These preferences are set in the gnome-control-center. Each application will respect the preferred browser and preferred email programs defined in there. I forgot where I found this tip recently but I can’t take credit for discovering it on my own.