Thanks for the useful library, however I'd like to uninstall it + remove this:

Warning: the rootCA-key.pem file that mkcert automatically generates gives complete power to intercept secure requests from your machine. Do not share it.

Where is rootCA-key.pem?

No, if you didn't run mkcert -uninstall the certificate is still trusted by your system. (This is only a problem if anyone obtained a copy of rootCA-key.pem.) Unfortunately if you deleted the whole mkcert folder mkcert -uninstall won't work now, so you have to remove the certificate manually from Keychain Access, as well as from the Firefox root store.