I located where my executable for firebird (mozilla-firebird) is, and found out it was a shell script...here's what's inside:
if [ "$2" = "tab" ]; then OPEN_IN=new-tab; fi
$MOZ_FIREBIRD_EX -remote "ping()" > /dev/null 2>&1
if [ $? -eq 0 ]; then
exec $MOZ_FIREBIRD_EX -remote "openURL($1,$OPEN_IN)"
if [ "$2" = "tab" ]; then $0 $1; exit 0; fi
exec $MOZ_FIREBIRD_EX "$@"
so basically it just defines whether to open a new window in tab or in a window. the next thing I did was to check out the MozillaFirebird which is an executable for this file...also a shell script (and a bit longer than this). it uses the run-mozilla.sh and at first glance it seemed that the run-mozilla.sh is the one which truly starts a mozilla-program....like it's the "core" of them all.
now what's interesting is the way it starts...I didn't spend too much time on it but it looked like it passes the program name to be run as a variable so could it be that after you've first started firebird, the variable is set to "MozillaFirebird" or something and it opens firebirds...but if you haven't, it uses just the normal mozilla?
in this case you just would need to find a way to set the variable before starting run-mozilla.sh or then change it in the script (which would disable the normal mozilla, if you don't specially tell run-mozilla to use it)...?
check out this:
$ locate MozillaFirebird
$ locate run-mozilla.sh
$ cat [where-ever-it-is]/MozillaFirebird
and after it look
$ cat [where-ever-it-is]/run-mozilla.sh
I think the variable passed to run-mozilla.sh is the key to this one