Issue with Headless selenium on CentOS 7 Minimal

Question

Required: I'm trying to run headless selenium (using PHPUnit) on fresh CentOS 7 minimal.

Problem: It seems I installed everything correctly but firefox is not launching and my selenium web driver stuck at 14:45:31.646 INFO - Launching Firefox.... With firefox command, I'm getting following output

[admin@EE ~]$ /usr/bin/Xvfb :1 -screen 0 1024x768x24 &
[1] 11932
[admin@EE ~]$ export DISPLAY=:1
[admin@EE ~]$ firefox
firefox:11936): Gtk-WARNING **: Locale not supported by C library.
    Using the fallback 'C' locale.
Xlib:  extension "RANDR" missing on display ":1".
Fontconfig warning: ignoring UTF-8: not a valid region tag
console.error:
  [CustomizableUI]
  Custom widget with id loop-button does not return a valid node
console.error:
  [CustomizableUI]
  Custom widget with id loop-button does not return a valid node
GLib-GIO-Message: Using the 'memory' GSettings backend.  Your settings will not be saved or shared with other applications.

Selenium web driver stucks at 04:22:53.835 INFO - Launching Firefox...

Can someone please suggest why selenium is not able to identify Firefox?