I recently found that my Selenium Java libraries were not working. Version 2.48.2 would open a Firefox window and then "stall" with no error within Netbeans nor Firefox itself before returning from
Exception in thread "AWT-EventQueue-0" org.openqa.selenium.remote.UnreachableBrowserException: Could not start a new session. Possible causes are invalid address of the remote server or browser start-up failure.
Build info: version: '2.53.1', revision: 'a36b8b1', time: '2016-06-30 17:32:46'
System info: host: 'MrComputer', ip: '192.168.0.101', os.name: 'Windows 7', os.arch: 'amd64', os.version: '6.1', java.version: '1.8.0_65'
Driver info: driver.version: FirefoxDriver
Problem Event Name: APPCRASH
Application Name: firefox.exe
Application Version: 184.108.40.20699
WebDriver driver = new FirefoxDriver();
Unfortunately Selenium WebDriver 2.53.0 is not compatible with Firefox
47.0. The WebDriver component which handles Firefox browsers (FirefoxDriver) will be discontinued. As of version 3.0, Selenium
WebDriver will have MarionetteDriver as the default running
implementation for Firefox tests.
Selenium WebDriver 2.53.1 has been released on 30th June 2016.
FirefoxDriver is working again with Firefox 47.0.1 as browser.
For Selenium 2.53.1 the Firefox version needs to be at least
47.0.1. Updating my Firefox did the trick.
EDIT: It also seems that version 47.+ was not released to the Ubuntu archives for a few weeks after the Windows version.