newbie question: Getting through XDM with a newly-installed applcation

["David A. Cobb" <superbiskit@cox.net>]

[NOTE: This is a re-send of a post to <debian-x@...>. With apologies]

Hi.
   Pardon the dumb question.  I've looked through what I can find and I
can't tell how what doesn't work differs from what does.
   I've installed packages mozilla-firefox and mozilla-thunderbird from
apt.  I also have ( in /opt/... ) versions un-tarred from the
mozilla.org nightlies.
   Of all of them, only the nightly thunderbird runs, and I can only
run that from my root account.
   If I try running without my root hat on, or if I run any of the
other three progies from anywhere, I get the following ( from
.xsession-errors -- so there may be irrelevant stuff here )

** Message: xfce4-panel: module mixer successfully loaded
** Message: xfce4-panel: module eyes successfully loaded
** Message: xfce4-panel: module tl successfully loaded

(xfce4-panel:6805): Gdk-CRITICAL **: file gdkdraw.c: line 412
(gdk_draw_rectangle): assertion `GDK_IS_GC (gc)' failed
Failed to retrieve terminal server from activation server
Failed to retrieve terminal server from activation server
selected locale: en-US
Xlib: connection to ":0.0" refused by server
Xlib: XDM authorization key matches an existing client!

(mozilla-thunderbird-bin:6904): Gtk-WARNING **: cannot open display:
+ moz_libdir=/usr/local/lib/thunderbird-0.6+


What do I need to do to get these things running?
Thanks,
--
David A. Cobb, semi-tetired Software Engineer (t-rex)