I cannot answer explicitly to your 2 questions (%u and %s), but maybe this can help.
I had this problem for a long time in RH9, using Firebird, mainly from tar.gz.
I tried many solutions using different parameter and remote one ... but none of them worked really good.
Since I installed firefox from Dag repositories, (the rpm file), launching firefox from Evolution mail just open a new tab (or open Firefox if there is no instance). At the beginning, I was thinking nothing happened, because Firefox did not get the focus, but when I checked my Firefox window, every click were opened in a new tab ;-) .
The rpm name is : firefox-0.8-1.rh90.dag.i386.rpm (I am using RH9)
The firefox launcher just lanched "firefox", meaning /usr/bin/firefox.
This is a shell script, so I think this is the one to have ;-)
I hope it helps .