This may be the same as bug 454677, but it started happening quite often to me only in the last few days, so I suspect it's different.

In the past four days, I've crashed three times with a BadIdChoice X error:
2008-09-28 12:42:13
2008-09-29 18:47:44
2008-10-01 10:39:24

The first two times the error looked like this:

The program 'firefox-bin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadIDChoice (invalid resource ID chosen for this connection)'.
  (Details: serial 158749545 error_code 14 request_code 155 minor_code 4)
  (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.)


(different serial the second time)

But the third time it gave me more information:

The program 'firefox-bin' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadIDChoice (invalid resource ID chosen for this connection)'.
  (Details: serial 150818044 error_code 14 request_code 155 minor_code 4)
  (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.)
Locking assertion failure.  Backtrace:
#0 /usr/lib/libxcb-xlib.so.0 [0x7f833992297c]
#1 /usr/lib/libxcb-xlib.so.0(xcb_xlib_lock+0x15) [0x7f8339922a15]
#2 /usr/lib/libX11.so.6 [0x7f833d7e4323]
#3 /usr/lib/libXrender.so.1(XRenderFreePicture+0x46) [0x7f833abd87c6]
#4 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/libthebes.so [0x7f83327bd8f4]
#5 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/libthebes.so(_moz_cairo_surface_finish+0x3b) [0x7f83327a51cb]
#6 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/libthebes.so(_moz_cairo_surface_destroy+0x58) [0x7f83327a5248]
#7 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/libthebes.so(_ZN11gfxASurface7ReleaseEv+0x36) [0x7f83327676c6]
#8 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libgkgfxthebes.so [0x7f832eed9c6f]
#9 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libgkgfxthebes.so [0x7f832eeda2ba]
#10 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libgkgfxthebes.so [0x7f832eee5544]
#11 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libgkgfxthebes.so [0x7f832eee584f]
#12 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/libxpcom_core.so(_ZN15nsCOMArray_baseD2Ev+0x3b) [0x7f833fd9b0eb]
#13 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libimglib2.so [0x7f832f30ed41]
#14 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libimglib2.so [0x7f832f30cccf]
#15 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libimglib2.so [0x7f832f31f178]
#16 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libimglib2.so [0x7f832f31d52c]
#17 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libimglib2.so [0x7f832f31820a]
#18 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/components/libimglib2.so [0x7f832f31ac35]
#19 /home/dbaron/builds/mozilla-central/obj/firefox-debugopt/dist/bin/libxpcom_core.so(PL_DHashTableFinish+0x73) [0x7f833fd96b43]


I'm going to attempt to turn the above stack into something more useful, although it will be a little tricky.