Below I've paseted the useful info for this host: OS, libxcb package, version numbers for X windows, java and firefox. The last 2 commands show 2 different ways to find the failed assertion. The 'Console' command is run from the command line, leading to a java failure. The 'firefox' version fails when I navigate the browser to
  http://java.com -> Do I have Java? -> Verify Installation

To be sure, I am running 32-bit versions of firefox and java.
Good luck,
Charlie

agi042:~ # cat /etc/SuSE-release
openSUSE 10.3 (X86-64)
VERSION = 10.3

agi042:~ # head -7 /var/log/Xorg.0.log

X Window System Version 7.2.0
Release Date: Wed Oct 24 14:18:27 UTC 2007
X Protocol Version 11, Revision 0, Release 7.2
Build Operating System: openSUSE SUSE LINUX
Current Operating System: Linux agi042 2.6.22.5-31-default #1 SMP 2007/09/21 22:29:00 UTC x86_64
Build Date: 24 October 2007

agi042:~ # rpm -q xorg-x11-libxcb xorg-x11-libxcb-32bit
xorg-x11-libxcb-7.2-51
xorg-x11-libxcb-32bit-7.2-51


agi042:~ # firefox --version
Mozilla Firefox 2.0.0.8, Copyright (c) 1998 - 2007 mozilla.org

agi042:~ # ls -l /usr/lib/firefox/plugins/libjava*
lrwxrwxrwx 1 root root 61 Oct 31 09:30 /usr/lib/firefox/plugins/libjavaplugin_oji.so -> /usr/lib/jvm/jre1.6.0_03/plugin/i386/ns7/libjavaplugin_oji.so

agi042:~ # /usr/lib/jvm/jre1.6.0_03/bin/ControlPanel
java: xcb_xlib.c:50: xcb_xlib_unlock: Assertion `c->xlib.lock' failed.
/usr/lib/jvm/jre1.6.0_03/bin/ControlPanel: line 135:  7550 Aborted                 ${java_home}/java -Djavaplugin.user.profile=${USER_JPI_PROFILE} -Xbootclasspath/a:${java_home}/../lib/deploy.jar ${_JAVA_VM_OPTIONS} com.sun.deploy.panel.ControlPanel

agi042:~ # firefox
java_vm: xcb_xlib.c:50: xcb_xlib_unlock: Assertion `c->xlib.lock' failed.
Could not read ack from child process
Plugin: Java VM process has died.
plugin: java process died due to signal 6
Could not start JavaVM!

VM did not start up properly