How to fix Selenium WebDriverException: The browser appears to have exited before we could connect?

Question

I have installed firefox and Xvfb on my centos6.4 server to use selenium webdriver.

But, when I run the code, I got an error.

from selenium import webdriver
browser = webdriver.Firefox()

Error

selenium.common.exceptions.WebDriverException: Message: 
'The browser appears to have exited before we could connect. The output was: None'

I read some related pages on stackoverflow and someone suggested to remove all files in tmp folder, so I did it. But, it still doesn't work.

Could anyone please give me a help?

Thank you in advance!

Edit

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.4/site-packages/selenium/webdriver/firefox/webdriver.py", line 59, in __init__
    self.binary, timeout),
  File "/usr/local/lib/python3.4/site-packages/selenium/webdriver/firefox/extension_connection.py", line 47, in __init__
    self.binary.launch_browser(self.profile)
  File "/usr/local/lib/python3.4/site-packages/selenium/webdriver/firefox/firefox_binary.py", line 64, in launch_browser
    self._wait_until_connectable()
  File "/usr/local/lib/python3.4/site-packages/selenium/webdriver/firefox/firefox_binary.py", line 103, in _wait_until_connectable
    self._get_firefox_output())
selenium.common.exceptions.WebDriverException: Message: 'The browser appears to have exited     before we could connect. The output was: None' 

Answer 1

for Googlers, this answer didn't work for me, and I had to use this answer instead. I am using AWS Ubuntu.

Basically, I needed to install Xvfb and then pyvirtualdisplay:

sudo apt-get install xvfb
sudo pip install pyvirtualdisplay

Once I had done that, this python code worked:

#!/usr/bin/env python

from pyvirtualdisplay import Display
from selenium import webdriver

display = Display(visible=0, size=(1024, 768))
display.start()

browser = webdriver.Firefox()
browser.get('http://www.ubuntu.com/')
print browser.page_source

browser.close()
display.stop()

Thanks to @That1Guy for the first answer