`mkcert -install` fails to install for Firefox on macOS

I am attempting to test rustls usage in actix-web with [this example](https://github.com/actix/examples/tree/master/rustls/). It recommended I use mkcert to set up a local CA.

It looks like mkcert was able to install it at the system level, but it gets errors when trying to install it for Firefox:

$ mkcert -install
The local CA is now installed in the system trust store! ⚡️
Installing in Firefox failed. Please report the issue with details about your environment at https://github.com/FiloSottile/mkcert/issues/new 👎
Note that if you never started Firefox, you need to do that at least once.

I have started Firefox before. The error occurs regardless of whether Firefox is started. The server can be started and accessed with `curl -k`, however Firefox returns `SEC_ERROR_UNKNOWN_ISSUER` and Chrome returns `NET::ERR_CERT_AUTHORITY_INVALID`.

System information:

- macOS 10.13.6
- Firefox 84.0.1 (64-bit)
- mkcert 1.4.3 (installed with `brew install --build-from-source`)