Firefox 'Cross-Origin Request Blocked' despite headers

If you don't have a 'real' certificate (and thus using a self-signed one), in Firefox you can go to:

Options > Privacy & Security > (scroll to the bottom) View Certificates > Servers > Add Exception...

There, fill in the location, eg: https://www.myserver:myport

I found that my problem was that the server I've sent the cross request to had a certificate that was not trusted.

If you want to connect to a cross domain with https, you have to add an exception for this certificate first.

You can do this by visiting the blocked link once and addibng the exception.

Turns out this has nothing to do with CORS- it was a problem with the security certificate. Misleading errors = 4 hours of headaches.

I came across this question having found requests in Firefox were being blocked with the message:

Reason: CORS request did not succeed

After pulling my hair out I found out that a newly installed Firefox extension, Privacy Badger, was blocking the requests.

If you come to this question after scratching your head, try checking to see what extensions you have installed to see if any of them are blocking requests.

See Reason: CORS request did not succeed on MDN for details.