Matthew Casperson discusses a common configuration issue encountered when using SOCKS proxies with Selenium WebDriver in Firefox, which results in a misconfiguration error. The error stems from the failure to define the SOCKS proxy version, leading to a failed assertion that an integer must be positive. Casperson explains that the error can be traced back to the file "chrome://marionette/content/session.js" bundled with Firefox, where the offending line of code is located. Although theoretically, the SOCKS proxy version can be set within the BrowserMobDecorator class to rectify this, practical issues with the WebDriver library may persist. Casperson emphasizes that understanding Firefox stack traces with URLs starting with "chrome://marionette/" allows developers to access these files directly in Firefox, aiding in debugging the root cause of errors. This post is part of a series focused on building a Selenium WebDriver test framework.