Ineffective "--with-user-appdir=NewDir" in Firefox-1.0.5 build ?

I'm trying to build Firefox-1.0.5 so that my (user) preferences
are stored in "~/.firefox" rather than "~/.mozilla".

I have the entry "ac_add_options --with-user-appdir=.firefox"
in my $MOZCONFIG file, which appears to set MOZ_USER_DIR=.firefox
correctly at build time.

The browser builds and installs without complaint on my FreeBSD 5.4-R
system using "gmake -f client.mk build install".

When I run the application, however, the browser creates *both*
~/.firefox and ~/.mozilla directories.
And all my preferences are stored under ~/.mozilla;
nothing gets put in ~/.firefox !

What gives ?

Where does the browser "know" about ~/.mozilla, anyway ?

Pointers/comments for getting this to work are appreciated.