After upgrading the following X11 libraries:

Current x11-libs libxt 1.0.1-1 X11 toolkit intrinsics library 2006-04-29
Current x11-libs libxvmc 1.0.2-1 X11 Video Motion Compensation extension library 2006-04-29
Current x11-libs libxxf86dga 1.0.1-1 X11 Direct Graphics Access extension library 2006-04-29
Current x11-libs libxxf86misc 1.0.1-1 X11 XFree86 miscellaneous extension library 2006-04-29
Current x11-libs libxxf86vm 1.0.1-1 X11 XFree86 video mode extension library 2006-04-29
Current x11-libs libxkbfile 1.0.3-1 X11 keyboard file manipulation library 2006-04-29
Current x11-libs libxkbui 1.0.2-1 X11 keyboard UI presentation library 2006-04-29
Current x11-libs libxmu 1.0.1-1 X11 miscellaneous micro-utility library 2006-04-29
Current x11-libs libxpm 3.5.5-1 X11 pixmap library 2006-04-29
Current x11-libs libxrandr 1.1.1-1 X11 RandR extension library 2006-04-29
Current x11-libs libxrender 0.9.1-1 X Rendering Extension client library 2006-04-29
Current x11-libs libxres 1.0.1-1 X11 Resource extension library 2006-04-29
Current x11-libs libxcursor 1.1.6-1 X cursor management library 2006-04-29
Current x11-libs libxdamage 1.0.3-1 X11 damaged region extension library 2006-04-29
Current x11-libs libxdmcp 1.0.1-1 X11 Display Manager Control Protocol library 2006-04-29
Current x11-libs libxevie 1.0.1-1 X11 EvIE extension library 2006-04-29
Current x11-libs libxext 1.0.1-1 X11 miscellaneous extensions library 2006-04-29
Current x11-libs libxfixes 4.0-1 X11 miscellaneous 'fixes' extension library 2006-04-29
Current x11-libs libxfontcache 1.0.2-1 X11 font cache library 2006-04-29
Current x11-libs libxi 1.0.1-1 X11 Input extension library 2006-04-29
Current x11-libs libdmx 1.0.2-1 X11 Distributed Multihead extension library 2006-04-29
Current x11-libs libfontenc 1.0.2-1 X11 font encoding library 2006-04-29
Current x11-libs libice 1.0.1-1 X11 Inter-Client Exchange library 2006-04-29
Current x11-libs libsm 1.0.1-1 X11 Session Management library 2006-04-29
Current x11-libs libxaw 1.0.2-1 X11 Athena Widget library 2006-04-29

both Firefox and Thunderbird will not launch and report the following error message:

The program 'Gecko' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadRequest (invalid request code or no such operation)'.
(Details: serial 744 error_code 1 request_code 152 minor_code 2)
(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.)