The Definitive Way?

Is there a definitive way to make Firefox aware of Thunderbird and 
Thunderbird aware of Firefox?

I mean, without fooling around with KDE, so a mail link in a web page 
will open Thunderbird and a web link in Thunderbird will open Firefox.

There is discussion, there are disagreements in those discussions, and 
Mozilla.org's FAQ doesn't really define precisely what to do.

Anybody actually *know* how to do this?

Thanks.