Self-signed certificates don't work on firefox

[theCalcaholic]

Expected behavior

Self signed certificates should show "CA not trusted" type errors on all browsers but work otherwise (after accepting the warning)

Actual behavior

Self signed certificates work on Chromium based browsers (tested on Chromium and Brave) but not on Firefox, where the following error message is shown:

An error occurred during a connection to localhost. Peer’s certificate has an invalid signature.

Error code: SEC_ERROR_BAD_SIGNATURE

    The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
    Please contact the website owners to inform them of this problem.

This issue does not occur when using a self signed certificate from a different source, e.g. generated using the following openssl command:

openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -sha256 -days 7 -nodes

Minimal configuration to reproduce the issue

./Caddyfile:

localhost:443 {
  bind 0.0.0.0
  tls internal

  respond "static response"
}

docker command:

docker run --rm -v "$PWD/Caddyfile:/etc/caddy/Caddyfile:ro" -p 443:443 caddy:2

Context

Caddy version: v2.8.4 h1:q3pe0wpBj1OcHFZ3n/1nl4V4bxBrYoSoab7rL9BMYNk=
Firefox version: 136.0 (64-bit) - Flatpak version

[bt90]

Could you check the TLS handshake using:

openssl s_client -connect localhost:443

and post the output?

[theCalcaholic]

Sure, here it is:

  $ openssl s_client -connect localhost:443
  Connecting to ::1
  CONNECTED(00000003)
  80B2A21DEF7F0000:error:0A000438:SSL routines:ssl3_read_bytes:tlsv1 alert internal error:ssl/record/rec_layer_s3.c:909:SSL alert number 80
  ---
  no peer certificate available
  ---
  No client certificate CA names sent
  ---
  SSL handshake has read 7 bytes and written 302 bytes
  Verification: OK
  ---
  New, (NONE), Cipher is (NONE)
  This TLS version forbids renegotiation.
  Compression: NONE
  Expansion: NONE
  No ALPN negotiated
  Early data was not sent
  Verify return code: 0 (ok)
  ---

EDIT: Here are the two certificates in question. One notable difference is, that caddy uses a (self signed) CA to issue the actual certificate, while I created a self signed certificate directly for my comparison - however, I've been doing the same procedure as caddy with openssl in the past and not encountered this problem.

caddy_internal_self_signed_cert.pem.txt
openssl_self_signed_cert.pem.txt