This is thanks to what is now my favoritest Mozilla extension yet: Mozex lets you specify external applications for viewing source, editing textboxes, mail, IRC, AIM, FTP, and news, as well as letting you specify a single universal handler for schemes that Mozilla does not know how to support. The external applications are called via a context menu, so you can always get the standard behaviour as well; it also lets you force the external application on mail:, irc:, telnet:, news:, and ftp: schemes.
Right now, I've configured it to use vim in an xterm for viewing source and for editing textboxes, xchat for irc links, and an xterm which connects to a remote host over ssh and starts mutt for mailto: links. The last one made me go "Oooh" the most.
Mozex is available for Unix (I'm using it in Linux) and Windows, for recent versions of Mozilla and Firebird.