Full backtrace from a debug build of todays mozilla-central/comm-central on Kubuntu 12.04 x86_64. Judging from the crash stats, it only happens on Ubuntu x86_64. It happens with the official Ubuntu packages as well as with the semi-official 64bit release builds from Mozilla. It does not, however, crash with the 32 bit release builds from Mozilla (I don't know about the i586 packages from Ubuntu).

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff5720ece in ldap_ld_free (ld=0x7ffff6cff730, serverctrls=0x0, clientctrls=0x0, close=1) at /home/username/src/comm-central/ldap/sdks/c-sdk/ldap/libraries/libldap/unbind.c:101
101                             nextlr = lr->lr_next;
(gdb) bt
#0  0x00007ffff5720ece in ldap_ld_free (ld=0x7ffff6cff730, serverctrls=0x0, clientctrls=0x0, close=1) at /home/username/src/comm-central/ldap/sdks/c-sdk/ldap/libraries/libldap/unbind.c:101
#1  0x00007ffff5720d27 in ldap_unbind (ld=0x7ffff6cff730) at /home/username/src/comm-central/ldap/sdks/c-sdk/ldap/libraries/libldap/unbind.c:60
#2  0x00007fffe393a955 in do_close () at ldap-nss.c:722
#3  0x00007fffe393bb5b in do_open () at ldap-nss.c:1701
#4  do_with_reconnect (base=0x7fffe3b4bce6 "ou=Users,OU=TU,OU=IAM,DC=ads,DC=mwn,DC=de", scope=1, filter=0x7fffffffbec0 "(&(objectClass=user)(uid=username))", attrs=0x7fffe3b4cb60, sizelimit=1, private=0x7fffffffc340, search_func=0x7fffe393acc0 <do_search_s>)
    at ldap-nss.c:2624
#5  0x00007fffe393d192 in _nss_ldap_search_s (args=0x7fffffffc3b0, filterprot=0x7fffe3b4f1c0 "(&(objectClass=user)(uid=%s))", sel=<optimized out>, user_attrs=0x0, sizelimit=1, res=0x7fffffffc340) at ldap-nss.c:3142
#6  0x00007fffe393e819 in _nss_ldap_getbyname (args=0x7fffffffc3b0, result=0x7fffffffc460, buffer=0x7ffff6ce8800 "nx", buflen=1024, errnop=0x7ffff7fd66b0, filterprot=<optimized out>, sel=LM_PASSWD, parser=0x7fffe393ea40 <_nss_ldap_parse_pw>) at ldap-nss.c:3509
#7  0x00007fffe393edb7 in _nss_ldap_getpwnam_r (name=<optimized out>, result=<optimized out>, buffer=<optimized out>, buflen=<optimized out>, errnop=<optimized out>) at ldap-pwd.c:247
#8  0x00007ffff70bc9ad in __getpwnam_r (name=0x7fffffffee06 "username", resbuf=0x7fffffffc460, buffer=0x7ffff6ce8800 "nx", buflen=1024, result=0x7fffffffc498) at ../nss/getXXbyYY_r.c:256
#9  0x00007fffee700e33 in g_get_any_init_do () at /build/buildd/glib2.0-2.32.3/./glib/gutils.c:765
#10 0x00007fffee70191d in g_get_any_init () at /build/buildd/glib2.0-2.32.3/./glib/gutils.c:916
#11 g_get_any_init_locked () at /build/buildd/glib2.0-2.32.3/./glib/gutils.c:923
#12 g_get_user_name () at /build/buildd/glib2.0-2.32.3/./glib/gutils.c:941
#13 0x00007fffe83a1c4d in gnome_client_instance_init (client=0x7ffff6c2bc00) at gnome-client.c:1385
#14 0x00007fffee9b3957 in g_type_create_instance (type=<optimized out>) at /build/buildd/glib2.0-2.32.3/./gobject/gtype.c:1892
#15 0x00007fffee9980b9 in g_object_constructor (type=<optimized out>, n_construct_properties=0, construct_params=0x0) at /build/buildd/glib2.0-2.32.3/./gobject/gobject.c:1849
#16 0x00007fffee999c02 in g_object_newv (object_type=140737333264896, n_parameters=0, parameters=0x0) at /build/buildd/glib2.0-2.32.3/./gobject/gobject.c:1632
#17 0x00007fffee99a1ec in g_object_new (object_type=140737333264896, first_property_name=0x0) at /build/buildd/glib2.0-2.32.3/./gobject/gobject.c:1542
#18 0x00007fffe83a3772 in gnome_client_new_without_connection () at gnome-client.c:1512
#19 0x00007fffe83a4540 in gnome_client_pre_args_parse (app=<optimized out>, mod_info=<optimized out>) at gnome-client.c:1143
#20 0x00007fffe7cca84e in gnome_program_preinit () from /usr/lib/x86_64-linux-gnu/libgnome-2.so.0
#21 0x00007fffe7ccb52e in ?? () from /usr/lib/x86_64-linux-gnu/libgnome-2.so.0
#22 0x00007fffe7ccb7ad in gnome_program_initv () from /usr/lib/x86_64-linux-gnu/libgnome-2.so.0
#23 0x00007fffe7ccb89f in gnome_program_init () from /usr/lib/x86_64-linux-gnu/libgnome-2.so.0
#24 0x00007ffff008b260 in nsNativeAppSupportUnix::Start (this=0x7ffff6cb1e80, aRetVal=0x7fffffffcdfa) at /home/username/src/comm-central/mozilla/toolkit/xre/nsNativeAppSupportUnix.cpp:500
#25 0x00007ffff007147a in XREMain::XRE_mainStartup (this=0x7fffffffcfa0, aExitFlag=0x7fffffffcf4e) at /home/username/src/comm-central/mozilla/toolkit/xre/nsAppRunner.cpp:3396
#26 0x00007ffff00736a3 in XREMain::XRE_main (this=0x7fffffffcfa0, argc=1, argv=0x7fffffffe418, aAppData=0x7ffff6c25700) at /home/username/src/comm-central/mozilla/toolkit/xre/nsAppRunner.cpp:3876
#27 0x00007ffff00739f1 in XRE_main (argc=1, argv=0x7fffffffe418, aAppData=0x7ffff6c25700, aFlags=0) at /home/username/src/comm-central/mozilla/toolkit/xre/nsAppRunner.cpp:4089
#28 0x0000000000402976 in do_main (exePath=0x7fffffffd310 "/home/username/src/comm-central/obj-x86_64-unknown-linux-gnu/mozilla/dist/bin/", argc=1, argv=0x7fffffffe418) at /home/username/src/comm-central/mail/app/nsMailApp.cpp:111
#29 0x0000000000402bf9 in main (argc=1, argv=0x7fffffffe418) at /home/username/src/comm-central/mail/app/nsMailApp.cpp:200


In a non-debug build I also got the warning
GLib-WARNING **: getpwuid_r(): failed due to: Verbindungsaufbau abgelehnt. ("Verbindungsaufbau abgelehnt" = connection refused)