Xorg signal 11's spontaneously

[Dennis Jacobfeuerborn]

In the last seven 10 days Xorg crashed three times with a signal 11 
spontaneously. This is the backtrace I found in the logfile:

Backtrace:
0: /usr/bin/Xorg(xf86SigHandler+0x87) [0x80b6d07]
1: [0x49a420]
2: /usr/lib/xorg/modules/libfb.so [0x134b81]
3: /usr/lib/xorg/modules/libfb.so [0x13c80c]
4: /usr/lib/xorg/modules/libfb.so(fbCompositeGeneral+0x855) [0x13a4e5]
5: /usr/lib/xorg/modules/libfb.so(fbComposite+0x1cd) [0x1433ad]
6: /usr/lib/xorg/modules/libxaa.so(XAAComposite+0x261) [0x81f001]
7: /usr/bin/Xorg [0x8156d06]
8: /usr/bin/Xorg [0x8153915]
9: /usr/bin/Xorg(CompositePicture+0x153) [0x8140a13]
10: /usr/bin/Xorg [0x814687c]
11: /usr/bin/Xorg [0x8143bd5]
12: /usr/bin/Xorg(Dispatch+0x19a) [0x80869ba]
13: /usr/bin/Xorg(main+0x485) [0x806e3e5]
14: /lib/libc.so.6(__libc_start_main+0xdc) [0x4cd7e4]
15: /usr/bin/Xorg(FontFileCompleteXLFD+0xbd) [0x806d721]

Fatal server error:
Caught signal 11.  Server aborting

All three crashes happened when I was loading a page in Firefox and after 
the nightly Firefox builds got switched to Cairo as their rendering backend 
so I wonder if Cairo could somehow trigger this.

I'm running Fedora Core Rawhide with the following Xorg packages:
xorg-x11-server-Xorg-1.0.99.901-5
xorg-x11-drv-nv-1.1.1-2

I'm using the open source nv driver with the AccelMethod set to EXA and the 
'load "dri"' and 'load "glx"' lines commented out.

Regards,
   Dennis