I had the same problem with Firefox 30 + Selenium 2.49 + Ubuntu 15.04.
It worked fine with Ubuntu 14 but after upgrade to 15.04 I got same RANDR
warning and problem at starting Firefox using Xfvb.
After adding +extension RANDR
it worked again.
$ vim /etc/init/xvfb.conf
#!upstart
description "Xvfb Server as a daemon"
start on filesystem and started networking
stop on shutdown
respawn
env XVFB=/usr/bin/Xvfb
env XVFBARGS=":10 -screen 1 1024x768x24 -ac +extension GLX +extension RANDR +render -noreset"
env PIDFILE=/var/run/xvfb.pid
exec start-stop-daemon --start --quiet --make-pidfile --pidfile $PIDFILE --exec $XVFB -- $XVFBARGS >> /var/log/xvfb.log 2>&1