Cannot Start The Driver Service On Http Localhost Selenium Firefox C »

Symptoms:
Exception mentions "Address already in use" or "Failed to bind to port". Sometimes the port number is explicitly 4444.

Cause:
Another process (another Selenium session, a zombie GeckoDriver, or a different application) is already using the port that GeckoDriver wants.

Fix:

To see exactly why the service failed, turn on debug logging for GeckoDriver:

from selenium.webdriver.firefox.service import Service
service = Service(
executable_path='geckodriver.exe',
service_args=['--log', 'debug']  # Forces verbose output
)
driver = webdriver.Firefox(service=service)
 Or let Selenium choose a random free port

You will see messages like:

If you see an error like “cannot start the driver service on http://localhost:xxxxx” when using Selenium with Firefox in C#, this note explains common causes and fixes. Restart your machine — a brute-force but effective

The “cannot start the driver service on http://localhost” error almost always comes down to:

Fix those, and your Firefox automation will run smoothly.

Did this help? Have another strange Selenium error? Let me know in the comments!

This error message (or variations of it) is one of the most common hurdles when setting up Selenium with Firefox in C#. It essentially means your C# code tried to launch the Firefox driver, but the driver process (geckodriver.exe) failed to start or crashed immediately.

Here is a review of the issue, broken down by cause, solution, and best practices.