But since Firefox 3.6, and newer updates of the addon fix, they would not work for me. Firefox 3.5 removed the open_external feature, which annoyed me, then a fix for it is now broken in FF 3.6.
I got fed up so I just made my own Firefox extension to fix it!
Restore Open_External 1.0
Restores the functionality of the 'browser.link.open_external' preference that was removed in Firefox 3.5+
(I have now learned how to make script-based FF extensions)
For me it's a user experience nightmare to remove such an option. For something to change, they should have promoted it to the preferences dialog instead (from only being visible in about:config).
It's very much required with multiple workspaces, too: you may not have Firefox running on the same workspace, and clicking a link in your mail app should open the link on the current workspace, of course.