If you want run your tests with Selenium without open the browser Firefox, I will write how you can do this:

Verify if you have the last version of Firefox.

1 – Install Firefox headless in Ubuntu

• Open the file: /etc/apt/sources.list and add the line:

  ppa:mozillateam/firefox-stable

• Open the terminal and run the commands:

   sudo apt-get update

   sudo apt-get install firefox

 

2 – Install Xvfb – the X Virtual FrameBuffer. This piece of software emulates the framebuffer using virtual memory which lets you run X-Server in machines with no display devices. This service is required to make browsers run normally by making them believe there is a display available.

• Open the terminal and run the command:

  sudo apt-get install xvfb

• Lets run the xvfb service in a display number which is less likely to clash even if you add a display at later stage. For this example, we will assume a display, 5. The parameter -ac makes xvfb run with access control off. The server should be running now.sudo Xvfb :5 -ac

 

3 – Start browser headlessly in Ubuntu

• Run the command:

  export DISPLAY=:5
  firefox

If there was no error on the terminal, then you have successfully running firefox headlessly in Ubuntu. The command should keep running until you kill the process by pressing ctrl+ C or similar. There wont be any output. Now, you can run selenium server as you will run in your local machine.

 

Font: http://www.installationpage.com/selenium/how-to-run-selenium-headless-firefox-in-ubuntu/