If you recently updated to selenium version 3.0 then you may face issue for lunching Firefox browser.

Like other browsers you have to use driver for Firefox as well if you are using selenium version 3.0 and above.

Mozilla has released geckodriver. You can download the latest executable on the GitHub page.

Download geckodriver according to your platform and simply put into system path and you are ready to use Firefox with Selenium 3.0

Read more about selenium 3.0 here.