Self signed certificate not working in Firefox - firefox

Question

For quite some time, I am trying to generate a self signed cert for viewing website locally in "real" https mode. It would be very important that this works across all OSes and also all Browsers (at least FF, Chrome and Safari).
Until now I created the cert using this guide: https://medium.com/#tbusser/creating-a-browser-trusted-self-signed-ssl-certificate-2709ce43fd15
Certs using this way work fine in all systems, except with Android (version 8 or greater), so in this case I cannot use some functions correctly (for example the Service Worker) when testing locally.
Now I have stumbled upon this guide, especially the answer here: https://stackoverflow.com/a/57684211/1794480
I have adapted the conf file a bit, and now the generated cert works in Android, but NOT in Firefox. The Error in FF I get is: MOZILLA_PKIX_ERROR_SELF_SIGNED_CERT. I also found an explanation for this error here: https://stackoverflow.com/a/59739121/1794480
Seems like basicConstraints = critical, CA:TRUE is the cuplrit here. However, the problem now is that this exact setting seems to be responsible for making this work on Android. (also see here: https://android.stackexchange.com/questions/237141/how-to-get-android-11-to-trust-a-user-root-ca-without-a-private-key/238859#238859)
Now, the big question is: is it possible to make this work for ALL systems, in this case including Firefox?
Here is the command I use: openssl req -config ./openssl.cnf -new -x509 -days 398 -out myServer.crt and here is the openssl.cnf:
[ req ]
default_bits = 2048
default_keyfile = myServer.key
default_md = sha256
default_days = 398
encrypt_key = yes
distinguished_name = req_distinguished_name
req_extensions = req_ext
x509_extensions = x509_ext
string_mask = utf8only
prompt = no
[ req_distinguished_name ]
commonName = myServer
emailAddress = info#myServer.de
[ x509_ext ]
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer
basicConstraints = critical, CA:TRUE
keyUsage = critical, digitalSignature, keyEncipherment, cRLSign, keyCertSign, dataEncipherment
subjectAltName = #alt_names
extendedKeyUsage = serverAuth
[ req_ext ]
authorityKeyIdentifier = keyid,issuer
basicConstraints = CA:FALSE
extendedKeyUsage = serverAuth
keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
subjectAltName = #alt_names
[ alt_names ]
DNS.1 = sub1.myServer
DNS.2 = sub2.myServer
Any help is greatly appreciated!