How to use Xvfb in Ubuntu 14.04 with/without RandR?

I try to run Unit-Tests with Selenium running Firefox on my Ubuntu 14.04 Server. And I'm using Xvfb as described in this blog to simulate a virtual display to show Firefox in.

But Xvfg somehow doesn't load/work with RandR. Because whenever I try this:

sudo Xvfb :10 -ac &
export DISPLAY=:10
firefox

I get an Xlib: extension "RandR" missing on display ":10"-Error.

I've also tried sudo Xvfb :10 -ac +extension RANDR, sudo Xvfb :10 -ac -extension RANDR and beacuse it supplies with "xrandr" also apt-get install x11-xserver-utils.

And my setup is a plain empty Ubuntu 14.04 Server with apt-get install xvfb firefox.

Can anyone please help me run Xvfb with or without RandR?

After hours of struggling the error is still there. But finally I've figured out, that it's more like a warning and that Firefox actually starts. Moreover, my Selenium-Tests are running.

The actual problem was, I used an old Selenium Version and the newest Firefox. And they didn't work well together.