Cannot Start The Driver Service On Http Localhost Selenium Firefox C [repack] May 2026

The error "Cannot start the driver service on http://localhost" is a common headache for C# developers using Selenium with Firefox. It typically means the GeckoDriver executable is unable to establish a local network connection to start its internal server. 🛠️ Root Causes Port Conflicts: Another process is using the default port. Loopback Issues: localhost isn't resolving to 127.0.0.1.

Version Mismatch: Incompatibility between the Selenium library, GeckoDriver, and the installed version of Firefox. The error "Cannot start the driver service on

Reason 5: Corrupted GeckoDriver or Firefox Installation

Symptoms:
Even with correct PATH and version, the service fails erratically. Loopback Issues: localhost isn't resolving to 127

to clear stuck instances that may be holding onto local ports. Environment Variable to clear stuck instances that may be holding

Remember the top three fixes:

Step-by-Step Troubleshooting Checklist

When you see "cannot start the driver service on http://localhost", run through this checklist in order:

Port Specificity: If the default port is blocked, you can manually assign a port using FirefoxDriverService.CreateDefaultService().Port = 9222.