Error when executing headless firefox through Selenium

Apparently this is caused because of incompatibility between Firefox 48 and Selenium(selenium extension is not signed in firefox 48, and firefox 48 only runs signed extensions). I just used chrome, as my use-case was not extremely browser-specific.

I solved this issue with Firefox. The steps are:

1. Downgraded Firefox from 49.0 to 46.0
2. Install XVFB by command sudo apt-get install xvfb
3. Run Selenium with the command xvfb-run java8 -jar selenium-project.jar

My Selenium version is 2.53.1 and Ubuntu 14.04.