DNS Booster is Firewalla's DNS Cache.
A DNS cache (sometimes called a DNS resolver cache) is a temporary database, maintained in memory, that contains records of all the recent visits and attempted visits to websites and other internet domains. After the initial visit to any website, the DNS lookup results will be returned from memory to speed up connections.
This feature has always been there to intercept and accelerate DNS request for all devices in the home network. As of 1.965, we decide to break the DNS cache function to be per device (rather than apply to all devices), hence you now see the DNS Booster function.