It has been reported that the X11 agent could terminate unexpectedly when running applications using libcairo version 1.12.0. Among these applications are firefox and thunderbird.

Downgrading libcairo solves the problem.

The backtrace can be different at each attempt, but it is always in the render extension code. Here is an example:

(gdb) bt
#0 0x00000000004ac268 in nxagentTrapezoids (op=3 '\\003', pSrc=0x40c1770, pDst=0x40c1970, maskFormat=<optimized out>, xSrc=1680, ySrc=31, ntrap=1,
traps=0x304aed0) at Render.c:1752
#1 0x000000000043790a in ProcRenderTrapezoids (client=0x3328ee0) at X/NXrender.c:1123
#2 0x000000000043131d in ProcRenderDispatch (client=<optimized out>) at X/NXrender.c:2519
#3 0x00000000004305d6 in Dispatch () at X/NXdispatch.c:747
#4 0x000000000040fea5 in main (argc=11, argv=0x7fff7a1a8168, envp=<optimized out>) at main.c:450