As reported in Bug 772446 comment 42 and attachment 658986 [details] centos6 leaktest is failing on shutdown in free(). The stack with changeset fdfaef738a00 is: /lib/libc.so.6(+0x6ed21)[0xd42d21] /lib/libc.so.6(+0x71713)[0xd45713] /builds/slave/m-cen-lnx-dbg/build/obj-firefox/dist/bin/libxul.so(free+0x3e)[0x21ab3e7] /builds/slave/m-cen-lnx-dbg/build/obj-firefox/dist/bin/firefox-bin(free+0x1b)[0x804b595] % addr2line -if -e libxul.so 0x1480a8b _cairo_xlib_visual_info_destroy /builds/slave/m-cen-lnx-dbg/build/gfx/cairo/cairo/src/cairo-xlib-visual.c:186 % addr2line -if -e libxul.so 0x14800f8 _cairo_xlib_screen_destroy /builds/slave/m-cen-lnx-dbg/build/gfx/cairo/cairo/src/cairo-xlib-screen.c:289 % addr2line -if -e libxul.so 0x1480bcd _cairo_xlib_display_destroy /builds/slave/m-cen-lnx-dbg/build/gfx/cairo/cairo/src/cairo-xlib-display.c:120 % addr2line -if -e libxul.so 0x1434d0b *INT_cairo_device_destroy /builds/slave/m-cen-lnx-dbg/build/gfx/cairo/cairo/src/cairo-device.c:341 % addr2line -if -e libxul.so 0x1480d05 _cairo_xlib_close_display /builds/slave/m-cen-lnx-dbg/build/gfx/cairo/cairo/src/cairo-xlib-display.c:241 /usr/lib/libX11.so.6(XCloseDisplay+0xab)[0x6dd7ffb] This seems related to running against a display/screen with depth 8. Switching to depth 24 avoids the problem. We could debug the issue in cairo with depth 8, but this is an unusual configuration, and only a shutdown crash. It would be better to test depth 24 anyway, as that is what our users will usually have. man Xvfb says "By default, only screen 0 exists and has the dimensions 1280x1024x8." so adding the arguments "-screen" "0" "1280x1024x24" to the Xvfb command seems the minimal-change remedy.