TLS 1.3 With X25519+Kyber512 in Firefox

(For the KEMTLS demo, switch to the kemtls branch.)

1. Run build.sh. Keep the default mozilla-unified as the source directory, and when asked to choose which version of Firefox to build, choose option 2: Firefox for Desktop.

2. Start Firefox by running ./mozilla-unified/mach run

3. In Firefox, load https://pq.cloudflareresearch.com.