Hi Salvador Sánchez Aroca! It sounds like you're facing a frustrating issue with CORS and authentication in Firefox. Let’s see if we can troubleshoot this together. Cross-Origin Credentials Rule: Firefox is stricter compared to Chrome when it comes to handling CORS with credentials. Even though you have Access-Control-Allow-Origin set correctly, make sure that both the request and response are handling credentials properly. For Access-Control-Allow-Credentials to be true, you need to ensure that Access-Control-Allow-Origin is not set to * — which appears to be correct in your case. Check Preflight Requests: Since you’re using POST, ensure your server correctly responds to the preflight OPTIONS request. Review the headers returned in response to the OPTIONS request to confirm they match your expectations. Use Firefox Debugging Tools: The Firefox Developer Tools can help you analyze network requests. Look at the "Network" tab to check the headers for both your OPTIONS and POST requests. This may reveal if any headers are missing. CORS and Path Matching: Check if there’s any mismatch between your origin (the referring domain) and the headers set on your server. Sometimes, discrepancies in subdomain or trailing slashes can cause issues. Browser Cache: Clear your browser cache, as old response headers might affect new requests. Test with Different Origins: If possible, try testing with a simpler setup to see if it’s an issue with your specific domain configuration. This can help isolate the problem. If the issue persists, consider setting up a minimal example or using tools like Postman or Insomnia to replicate the request. This can help determine if it’s a browser-specific issue or a problem with your server setup. I hope this helps, and best of luck with your project! Cheers!