Just after opening some URL[*] with Iceweasel, the system froze
for several seconds (I assume that it was constantly swapping),
then the X server crashed. This is the first time I got such a
problem.

[*] http://dailygeekshow.com/2013/10/17/13-photographies-qui-marqueront-a-jamais-notre-histoire/

The .xsession-errors.old file contained:

[... (old messages)]
libGL error: failed to load driver: nouveau
libGL error: Try again with LIBGL_DEBUG=verbose for more details.
libGL error: failed to load driver: swrast
libGL error: Try again with LIBGL_DEBUG=verbose for more details.
out of memory: 0x00000000000FF000 bytes requested
Segmentation fault
/usr/bin/xterm: fatal IO error 11 (Resource temporarily unavailable) or KillClient on X server ":0"

then other errors from X clients (xterm, emacs...) due to the fact that
the X server died.

Note that I start Iceweasel with "ulimit -v 4096000" so that it
doesn't take too much memory. The system has 4 GB physical memory
and 9.5 GB swap. Before the crash, there was plenty of memory
left, so that if the system ran out of memory, this came from X.

I couldn't see any core dump.

Please attach the X log from the crash.

Cheers,
Julien

Unfortunately it wasn't kept, because when I logged in just after
the crash, Iceweasel froze most of the parts of the window manager
(I could only switch the desktops) and I rebooted the machine.