Here's the backtrace from recent crash.  I had set a breakpoint at gdk_x_error() and run FF with --sync.

Gdk-ERROR **: The program 'firefox-bin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadAlloc (insufficient resources for operation)'.
  (Details: serial 199057 error_code 11 request_code 53 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
aborting...

Program received signal SIGABRT, Aborted.
0xba25f22f in kill () from /usr/lib/libc.so.12
(gdb) bt
#0  0xba25f22f in kill () from /usr/lib/libc.so.12
#1  0xba2fb338 in abort () from /usr/lib/libc.so.12
#2  0xba5f5b1d in g_logv () from /usr/pkg/lib/libglib-2.0.so.0
#3  0xba5f5b3b in g_log () from /usr/pkg/lib/libglib-2.0.so.0
#4  0xba8cafa9 in gdk_error_trap_push () from /usr/pkg/lib/libgdk-x11-2.0.so.0
#5  0xba471c73 in _XError () from /usr/pkg/lib/libX11.so.6
#6  0xba4736ee in _XReply () from /usr/pkg/lib/libX11.so.6
#7  0xba46da95 in XSync () from /usr/pkg/lib/libX11.so.6
#8  0xba46dc19 in _XSyncFunction () from /usr/pkg/lib/libX11.so.6
#9  0xba45203b in XCreatePixmap () from /usr/pkg/lib/libX11.so.6
#10 0xba8cbe5a in gdk_pixmap_foreign_new () from /usr/pkg/lib/libgdk-x11-2.0.so.0
#11 0xba89ba8d in gdk_pixmap_new () from /usr/pkg/lib/libgdk-x11-2.0.so.0
#12 0xbb94a74a in gfxPlatformGtk::CreateOffscreenSurface () from /usr/pkg/lib/firefox/libxul.so
#13 0xbb93ab33 in gfxPlatform::OptimizeImage () from /usr/pkg/lib/firefox/libxul.so
#14 0xbb815be6 in non-virtual thunk to nsPrintSession::Release() () from /usr/pkg/lib/firefox/libxul.so
#15 0xbfbfd7a8 in ?? ()
#16 0x08630600 in ?? ()
#17 0x0a175400 in ?? ()
#18 0x00000001 in ?? ()
#19 0x0abdfa9c in ?? ()
#20 0x0a0cc980 in ?? ()
#21 0x0b49ee00 in ?? ()
#22 0x0b49eed0 in ?? ()
#23 0x0b49eecc in ?? ()
#24 0xfffeefb4 in ?? ()
#25 0xbfbfd7e8 in ?? ()
#26 0xbbb9ed20 in ?? () from /usr/pkg/lib/firefox/libxul.so
#27 0x096ff000 in ?? ()
#28 0x0c212b00 in ?? ()
#29 0xbfbfd7d8 in ?? ()
#30 0xbb9224cb in NS_InvokeByIndex_P () from /usr/pkg/lib/firefox/libxul.so
#31 0xbb9224cb in NS_InvokeByIndex_P () from /usr/pkg/lib/firefox/libxul.so
#32 0xbb0b53c9 in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#33 0xbb0c1a7e in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#34 0xbb0c35ac in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#35 0xbb0bd8f1 in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#36 0xbb0b7a5f in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#37 0xbafcb0cc in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#38 0xbb02a3d1 in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#39 0xbafb21c5 in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#40 0xbafb2c0d in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#41 0xbb8dca3f in NS_StringContainerInit2_P () from /usr/pkg/lib/firefox/libxul.so
#42 0xbb8f449c in NS_GetComponentManager_P () from /usr/pkg/lib/firefox/libxul.so
#43 0xbb8bc5d3 in GetSecurityContext () from /usr/pkg/lib/firefox/libxul.so
#44 0xbb802c8f in JSD_DebuggerOnForUser () from /usr/pkg/lib/firefox/libxul.so
#45 0xbb69f9b0 in DumpJSEval () from /usr/pkg/lib/firefox/libxul.so
#46 0xbaf38fca in XRE_main () from /usr/pkg/lib/firefox/libxul.so
#47 0x0804900a in _rtld_setup ()
#48 0x08048d44 in ___start ()
#49 0x08048ca7 in _start ()