Selenium - Could not start Selenium session: Failed to start new browser session: Error while launching browser

Chances are this problem is caused by an already-running instance of the Selenium server. The new instance needs to listen on the same port number, but can't, because the port is already in use.

Let's say your Selenium server is configured to start on port 4444. Determine if the port is in use using the 'netstat' command:

On Windows: netstat -an | find "4444"

Expect to see output like this:

  TCP    0.0.0.0:4444           0.0.0.0:0              LISTENING
  TCP    [::]:4444              [::]:0                 LISTENING

On Linux, use: netstat -anp | grep 4444

(No Linux box to hand, so can't show example output!)

If you see any output, you need to kill the process that's listening on the port that Selenium wants to use. On Windows, use netstat -anb to find the process name (it'll be listed after the line specifying the port number). Kill it using the Task Manager. On Linux, the process PID and name will be listed by the command above - kill it using kill <PID>.

If you're using the last version of Selenium RC (after 1.0) you should change the following:

setUp("https://www.google.com/", "*chrome");

for

setUp("https://www.google.com/", "*firefox");

If this doesn't work, try creating a separate firefox profile and using that one:

http://seleniumhq.org/docs/05_selenium_rc.html#specifying-the-firefox-profile

I know this might sound silly but are you sure you have given selenium.start() in your code? Beginners can make this mistake.