Selenium Exception: Unable to Delete File (parent.lock)

Problem: When using FireFox for Selenium tests, Selenium RC is throwing exceptions with the error message “Selenium.SeleniumException: Failed to start new browser session: Unable to delete file” with a reference to a “parent.lock” file in your temp folder. Cleaning up FireFox profiles doesn’t help.

Solution: You may be using an older version of the selenium-server.jar. Go grab the latest download of Selenium RC. At least that’s what fixed it for us…