Firefox error for Istio client certificate authentication


I have an issue with Firefox when I try to enable the MUTUAL TLS mode in Istio. Namely, I followed these instructions here, and created self-signed certificates for my web pages. The issue is that I can only open one of the pages in Firefox and when I try to open the second one which has a different sub-domain gives the following error: SEC_ERROR_REUSED_ISSUER_AND_SERIAL
I tried to recreate the certificate with a different serial but it still won’t work. I tried using this workaround but it also doesn’t work in my case. The thing is that using cURL or Safari everything works great, I just can’t get it to work with Firefox.
Does anyone have an idea of what could I be doing wrong maybe?