Cannot Start The Driver Service On Http Localhost Selenium Firefox C [ 1000+ COMPLETE ]

Selenium automatically launches the GeckoDriver executable. This driver creates a local HTTP server on a dynamically assigned port (e.g., http://localhost:65175/ ). Your test code communicates with this server via HTTP, and the server then translates your commands into actions performed by the Firefox browser.

Download the executable and place it in a known folder. Code: Specify the path directly in your C# code:

In the vast majority of cases, the problem is resolved by one of these three actions:

The “Cannot start the driver service on http://localhost” error in Selenium Firefox with C# is almost always caused by one of the following issues, listed in order of likelihood: Selenium automatically launches the GeckoDriver executable

The error is almost never a Selenium bug—it is a local environment misconfiguration. By understanding the roles of GeckoDriver, Firefox, and localhost ports, you can systematically eliminate the causes.

If a previous automation session crashed or closed improperly, a zombie geckodriver process might still be clinging to the communication port. Open ( Ctrl + Shift + Esc ).

options = Options() options.add_argument("--headless") driver = webdriver.Firefox(options=options, service=service) Download the executable and place it in a known folder

Open C:\Windows\System32\drivers\etc\hosts in Notepad as an Administrator. Ensure that the line 127.0.0.1 localhost is not commented out with a # . 4. Clean Ghost Processes via Task Manager

If the error persists, try running your test without a proxy connection first to isolate the issue.

If you are running your C# Selenium tests in a CI/CD pipeline (such as Azure DevOps, GitHub Actions, or Docker containers), the lack of a display monitor or specific environment arguments will cause the driver service to fail immediately. If a previous automation session crashed or closed

The most common trigger for this error is a mismatch between your installed Firefox version and the geckodriver executable, or Selenium simply cannot find the executable file on your system's PATH.

If you frequently stop your test executions abruptly via the IDE (e.g., stopping the Visual Studio debugger mid-test), the driver.Quit() method is bypassed. This leaves orphan geckodriver.exe and firefox.exe processes running invisibly in the background. Eventually, these processes can lock up system resources or conflict with new port allocations.

using (var driver = new FirefoxDriver(service, options, TimeSpan.FromSeconds(60)))

The Selenium Manager will automatically download the appropriate GeckoDriver version for your system and Firefox version.