firefox – Client authentication on Apache server doesn’t work

I setup a simple Apache 2 server that uses a secure HTTPS connection. I created my own self-signed Root-CA-certificate and Sub-CA-certificate to create the server certificate and installed that on the server and that works fine.

I also wanted to enable client authentication so that users could only access the website when they had a valid user certificate. So I also enabled the SSLVerifyClient require option as well.

I used the Sub-CA to create a client certificate. I exported the certificate together with the private key used to create it in the p12 format and installed it in Firefox. However, accessing the website doesn’t work. I get this error:

Error

I troubleshooted a bit and somewhere it said that it might be because Firefox doesn’t seem to send the installed certificate at all? But I have no idea how to fix this.

Does anyone have an idea how to fix this?